add support for one part of App Annie's intelligence api

* AppAnnie::Intelligence.top_app_charts
appeasing some of houseci's recommendations
specify rspec version in gemspec
specify github source in README gemfile example, remove gem install example cause it wont work
AppAnnie::BadResponse class was used but never created
create AppAnnie::ErrorReponse class

* remove some code duplication with error handling
* AppAnnie::ErrorResponse.raise_for(response) will handle
all non-200 errors.
Merge pull request #2 from dylanjha/master

add support for one part of App Annie's intelligence api