[#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)
[add] ability to specify :branch in Podfiles and Podspecs
[fix] small syntax error
[Platform] Require name for initialization.
Merge pull request #321 from CocoaPods/platform-clean

[Platform] Require a name on initialization
Improve error message for missing local pod. Closes #323.
List all the specifications of a local pod.
[LocalPod] Deleted unnecessary #subspecs method.
Merge branch 'master' into develop

* master:
  List all the specifications of a local pod.
  Improve error message for missing local pod. Closes #323.
[Docs] Improved documentation of LocaPod.
[LocalPod] Support for header dir specification in subspecs.

The change is small refactoring to make clear which methods
return relative paths.
Removed forgotten require for `ruby-prof`.
[LocalPod] Fix for unavailable #select! in 1.8.7.
[LocalPod] Fix #all_specs_public_header_files.
[LocalPod] Sandbox header files.
[LocalPod] Specs and fix all_specs_public_header_files.

Introduced Chameleon fixture.