Add option to hide git output

controls the output option in https://github.com/CocoaPods/CocoaPods/blob/680e66dd350fcaed188e2fac0585c9c664b3919f/lib/cocoapods/sources_manager.rb#L156
Merge branch 'master' into hide_output_option
Merge pull request #84 from clayallsopp/hide_output_option

Add option to hide git output