Add a gemspec. Fixes #6

* Add name
* Add version (This will need to be updated on any changes after the
push to RubyGems.org)
* Add a short cummary
* Add a better desciption of the Gem (This could be improved)
* List @xpac27 and @glorieux as the authors
* Add @glorieux's email for any contact regarding the gem (This could
be updated to a mailing list)
* Add the gem's files using Rake
* Add the future homepage of the gem on rubygems.org

https://github.com/Leetchi/leetchi-api-ruby-sdk/issues/6