Disable rubygems when running gitsh.

There are currently some load errors related to an installer bug (see issue
 #178).

When rubygems is enabled it modifies require, which makes these bugs worse
by failing more slowly and with less helpful output (e.g. on my Mac OS X
system with Homebrew Ruby, it installs the Nokogiri gem before failing).
Include lib/gitsh/*/*.rb files in installation.

The previous install mechanism would only install files from `lib/gitsh`; it
excluded files in subdirectories. c1dc545c21a8037f47d71bf8ca01e667433f6d6e
moved the command classes into a subdirectory under `lib/gitsh`, and
subsequently these files weren't included by the installer.