Tab complete file paths containing spaces

* Adds support for tab completing file names with spaces.
* Handles quoted arguments correctly (i.e. spaces are not escaped within quotes)

Fixes #83 
I've started integrating the custom Readline code on the `gb-custom-readline-extension` branch.

So far I've got a working C extension which creates an empty `Gitsh::LineEditor` module, which is properly configured and built by the `./configure && make` dance. Running `make install` will install it globally for the selected Ruby version without doing any extra work, which means that other programs could theoretically `require 'gitsh/line_editor`, but that's probably fine.

All that remains now is to make it, y'know, do things, and to update the `configure.ac` or `extconf.rb` scripts to ensure we have access to all of the relevant line editing features.