Namespace linker flags using PODS_LDFLAGS

This is my first attempt to namespace OTHER_LDFLAGS as PODS_LDFLAGS in the xcconfig files.

I wanted to make the change post-aggregation of libraries, frameworks etc, so it's a bit of a hack at the moment that happens just before it returns.

I've updated the unit tests for this behaviour. I might need someone to point me in the right direction for getting the integration tests to pass. It seems there is a set of input Podfiles and output Pods.xcconfigs I would need to update. Is that correct?

I've based some of this on a conversation that started on the mailing list (https://groups.google.com/forum/?fromgroups=#!topic/cocoapods/hu1UsE1TyCg) and also relates to this issue https://github.com/CocoaPods/CocoaPods/issues/911