Closes https://github.com/CocoaPods/CocoaPods/issues/2586.

[Downloader] ensure that a downloader failure results in a DownloaderError so the Downloader can catch it

Helpful for https://github.com/CocoaPods/cocoapods-downloader/pull/36

Modify add_xcode_base_configuration to respect existing base configurations

Thanks @alloy!

Clarify why .git/ is removed from comparison

We have only so much time in a day, it’s time to take stronger stances on support.

configurations:
 - Do not change the base configuration when it is already set
 - Notify the user when there is an existing base configuration

Change string quoting from double to single quotes.

Split lines longer than 80 characters into multiple lines.

Quote initial string fragment with single quotes rather than double quotes.

[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[Outdated] Refine pod outdated to show what pod update would do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[Outdated] Take into account the sources specified in Podfile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