Fixed a bug with :head download and no prior clone

The post install hook is not so rare and there is no need for
this message anymore.

Full message: The `post_install' hook is reserved for edge cases`

Some repositories require that submodules be initialized and updated to
be properly built.  This is now done during the download phase for every
repository.

Added --local-only to pod push

Add Pod::Podfile::Informative error which show lines in Podfile

Added a --local-only option to pod push so that developers can push locally and test before pushing to a remote.

raise informative error message when user supplied invalid platform

Make PodsDummy class unique to each target

When generating the PodsDummy class, make that class unique to each target by using the Target Definition's label.

Closes #398.

Unfortunately, OS X >= 10.7 ships with 1.3.6 :(