add usage
handle errors when gem was not found
refactorings of Gemrat module

   extract methods from Gemrat#fetch_gem
   encapsulation
   remove unnecessary specs
stub Rubygems request and enjoy fast tests
remove unnecessary runtime dependencies
remove Gemfile.lock

http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
add the default rake task
Merge pull request #6 from shime/master

some improvements