Man pages should be the canonical source of hub documentation. People
should use `man help hub` instead of viewing the README, which might not
even correspond to the version they have installed.

This ends the confusing man page build system in which parts of the
README are injected into `hub.1.ronn` source file, deletes the Rakefile
and removes rake from gem bundle.

The examples are deleted because they are now inlined with individual
command documentation. So did the documentation for all hub commands get
deleted from main man page in favor of individual man pages.

However, this makes main man page have absolutely no listing of which
commands are available. This will get addressed as we go forward.

In the future, individual man pages will be generated prior to release.

This mimics `git` behavior.

Instead of displaying plain text help, `hub help <command>` will now
search for man pages to display.

* If `man` is available, hub first tries to locate the local
  `../man/hub-<command>.1` file relative to the `hub` executable. If
  found, open it with `man`. If not, execute `man 1 hub-<command>`.

* Otherwise, resort to displaying plain text help.

For compatibility with git, `hub help checkout` will display the help
for git-checkout and there will be no information about hub extensions
to this command.

Now with `hub help hub-checkout`, hub will print the help text from
hub's extension to git-checkout.

- No more `c.Short` property. Instead, the first line of `c.Long`
  property is considered a short command description.

- The `c.Usage` text can now contain multiple lines.

- The new `c.Synopsis()` method renders usage synopsis for humans:

    Usage: hub my-command --arg
           hub my-command --alternative-arg

- The new `c.HelpText()` method renders synopsis + full help text.

Don't corrupt the PATH if longer than 1024 characters on Windows

Help text improvements

Improve detecting local clone resources

Fixes #862

In `hub clone NAME`, "NAME" was previously considered a local resource
if a directory of the same name existed, and the clone command was left
unchanged.

That worked for a while, but some users were surprised that they
couldn't clone their repo named "NAME", only to discover that there was
an unrelated directory "NAME" in the current working directory.

Also, a git bundle is a valid cloneable resource, but is a file and not
a directory.

This refines the detection of cloneable resources. "NAME" is considered
to be locally cloneable if one of the following is true:

- "NAME/.git" exists
- "NAME" is a bare git repo
- "NAME" is a file whose first line includes "git bundle"

Add new flag to "release" for specifying commitish

Avoid "user: Current not implemented" error

Detect current username without resorting to cgo

Sometimes when hub is cross-compiled, `user.Current()` won't work on
certain systems. Use an alternative way of obtaining the user's home
directory.

- git+ssh://github.com/...
- ssh://github.com:22/...
- git@github.com:/...

Fixes #876

In instances where the `SHELL` environment variable is not set, nor is the `-s` parameter set it is not clear what action should occur.  Clarify the error message to make it obvious.

Closes #1065

Untested.