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.