[Source] Add the notion of a source URL
[RuboCop] Prefer $CHILD_STATUS from the English library over $?.
[FileSystemDataProvider] chomp the url
[RuboCop] Update .rubocop_cocoapods.yml
[Documentation] Update documentation for Podfile#source to reflect the fact that the parameter is now a URL string
[GitHubDataProvider] Remove the GitHub Data Provider, as it is unused

See https://github.com/CocoaPods/Core/pull/174 for the discussion.
[Changelog] Add entry for Source#url
Merge pull request #174 from CocoaPods/source-url

Give Sources and SourceProviders a url attribute