add a gemspec so gem can be used directly from git

One more pull request :-)
This one adds a gemspec file so that the gem can be used directly from git instead of having to use releases from rubygems.org (bundler supports using git as a gem source). This is a common practice in use by many gems so that people can get fixes and features before the official release.  
In all the pull requests I've been submitting, I've been putting this gemspec in my own repo so that I can use all the changes without having to manually build the gem and publish it somewhere.