[Xcodeproj] Don't promote warnings to errors
[Specs] Fix issue related to REST constant
[xcconfig] Only quote the path to the -isystem option.

Conflicts:
	lib/cocoapods/generator/xcconfig/aggregate_xcconfig.rb
	spec/unit/generator/xcconfig/aggregate_xcconfig_spec.rb
[Xcconfig] Changelog and Integration tests for -isystem option patch
[Downloader] Improved escaping of paths
[Downloader] Improved escaping of paths 2
[Gemspec] Bump Xcodeproj
[Validator] Check the homepage of the spec

Thanks @dlackty
[Specs] Speed-up Command::Lib
Release 0.31.0