[ResolverSpec] Add spec covering the exclusion of subspec dependencies of incompatible platforms when there's only one target platform

Done by using AFNetworking 2.4.1 and its UIKit subspec (iOS only) on an OS X platform

Fixes #2752.

Update for https://github.com/CocoaPods/CocoaPods/commit/7840787e05afb1da24bcfafa4a62f0de41ebf07a.

Revert "[Analyzer] Allow using CocoaPods with only externally-sourced dependencies in a   `Podfile` without specifying a source or displaying a warning"

This reverts commit 2cc9f01d.

Follow-up to https://github.com/CocoaPods/CocoaPods/commit/7840787e05afb1da24bcfafa4a62f0de41ebf07a.

Conflicts:
	lib/cocoapods/installer/analyzer.rb

Update for https://github.com/CocoaPods/CocoaPods/commit/3a61c4a265d97c41bff17a2c3aeea878f228041b.

[Analyzer] Improve handling of different types of duplicate dependencies

Closes https://github.com/CocoaPods/CocoaPods/issues/2753.

After much deliberation, the conclusion is that there is no good
techincal reason to enforce this, after all, and as such the decision
to deprecate it has been reverted.

https://github.com/travis-ci/travis-ci/issues/2918

[Analyzer] Allow using CocoaPods with only externally-sourced dependencies in a   `Podfile` without specifying a source or displaying a warning

Closes https://github.com/CocoaPods/CocoaPods/issues/2735.

[Sandbox] Symlink vendored framework headers into the public headers dir.

Fixes #1992.

For #1249

Improved sanitizing of configuration names which have a number prefix.