Validation of subspecs

The validation done by `pod spec lint` and `pod lib lint` commands
now checks that all subspecs build independently.

This new behavior can be disabled with `--no-subspecs`.

The validation of a single subspec can be done checked with `--subspec=NAME`.
Merge branch 'add-subspecs-lint'

* add-subspecs-lint:
  Validation of subspecs

Conflicts:
	lib/cocoapods/command/lib.rb