[UserProjectIntegrator] Allows the use of the `GCC_PREPROCESSOR_DEFINITION` flag `${inherited}`  without emitting a warning

Closes https://github.com/CocoaPods/CocoaPods/issues/2577

[Outdated] Refine pod outdated to show what pod update would do

Didn't work before because `instance_eval` changes the scope of the block

[ValidatorSpec] Update 'does filter InputFile errors completely' spec to pass source URLs to Validator

[Outdated] Take into account the sources specified in Podfile

[Validator] Allow the validator to take specific sources

Allows customizable source for `pod spec lint` and `pod lib lint`, with both defaulting to `master`.

Fixes #2542

Since this would produce code which isn't compliant to our own
styleguide/rubocop

See https://github.com/CocoaPods/CocoaPods/issues/2526#issuecomment-56986262

[CHANGELOG] Fix typo.

This is no longer relevant with trunk

Closes #2519

Minor README cleanup

Thanks @alloy