Making GreenButton Parser accessible as a gem.

* Adding a gemspec file to make it accessible as a gem
* Renamed parser.rb to greenbutton.rb to make it easier to resolve module name to source file. Referece: http://stackoverflow.com/questions/11525350/uninitialized-constant-error-when-including-a-module
* Tested it as local path gem library as follows:
gem 'greenbutton', :path => '/home/kunkunur/dev/prototype/greenbutton'
* The below should work as well
gem 'greenbutton', :path => 'https://github.com/sandeepkunkunuru/greenbutton'