[#221] Pod::Spec#public_header_files accepted and validated by `pod spec lint`
[#221] Pod::Sandbox#build_header_storage and Pod::Sandbox#public_header_storage

Those methods return Pod::HeaderStorage instances, managing respectively build headers and public headers.

Those objects replace and encapsulate former Pod::Sandbox#headers_root, add_header_file, add_header_files and header_search_paths methods.

For instance, instead of sandbox.header_search_paths, write sandbox.build_header_storage.search_paths.
[#221] HEADER_SEARCH_PATHS value comes from the new variable PODS_HEADER_SEARCH_PATHS. PODS_HEADER_SEARCH_PATHS is set to public headers search paths in Pods.xcconfig, and to build headers search paths in Pods.xcodeproj.
[#221] code cleanup
[#221] robustness against nil file lists (such as public_header_files)
Added a --local-only option to pod push so that developers can push locally and test before pushing to a remote.
Add Pod::Podfile::Informative error which show lines in Podfile (Re: #403)
Merge pull request #408 from CocoaPods/informative

Add Pod::Podfile::Informative error which show lines in Podfile
Merge pull request #405 from dlvenable/pod-push-no-remote

Added --local-only to pod push
Update bundle.
Merge branch 'master' of github.com:CocoaPods/CocoaPods
Run git submodule update after initial clone

Some repositories require that submodules be initialized and updated to
be properly built.  This is now done during the download phase for every
repository.
Make tests green.
[Linter] Removed message post install hook note.

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`
[Changelog] Update.
Update example project Podfiles
Raise an exception if a downloader doesn't support :head.
Clean pods *after* resolving and activating. Fixes #414.
[CHANGELOG] Update.
[CHANGELOG] Update.