Fix :head option for Pods from HTTP source.
Fail with a helpful error if :head is unsupported.
Test that helpful error message is actually displayed.
Stub out the actual downloading.
Merge branch 'fix_head_download_option' of https://github.com/neonichu/CocoaPods into neonichu-fix_head_download_option

Conflicts:
	spec/unit/installer/pod_source_installer_spec.rb