[#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.

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.

Closes #105, #121, #215.

Still need to do it at the target level too.

[#212] Check for deployment target

- Allow to specify the deployment target after initialization

* master:
  [Installer] Don't generate documenation if already installed
  [Command::Spec::Linter] Fix - Skip documentation generation
  [Config#doc] Renamed to generate_docs
  [Command::Spec::Linter] Bug fix.
  [# 242 Command::Spec::Linter] Refactoring
  [#246 | Resolver] Validate plaforms against each target.
  [Resolver] Check target plafroms instead of podfile.

Conflicts:
	lib/cocoapods/resolver.rb

It checks that the documenation exist for a given version of a pod
and in that case it doesn't generate it again.

- moved Specification#validate! to Linter
- fixed FileList bugs
- cleaned up

Related to [#239]

raising.