Closes #423.

* https://github.com/groue/CocoaPods:
  [#221] robustness against nil file lists (such as public_header_files)
  [#221] code cleanup
  [#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] Pod::Sandbox#build_header_storage and Pod::Sandbox#public_header_storage
  [#221] Pod::Spec#public_header_files accepted and validated by `pod spec lint`

Conflicts:
	lib/cocoapods/command/spec.rb
	lib/cocoapods/installer/target_installer.rb
	lib/cocoapods/local_pod.rb
	lib/cocoapods/sandbox.rb
	lib/cocoapods/specification.rb
	spec/functional/command/spec_spec.rb
	spec/unit/local_pod_spec.rb

Better spec source validation

In case of project have no Frameworks group, raise a meaningful error.

* 'master' of github.com:CocoaPods/CocoaPods:
  [Changelog]

* 'master' of github.com:CocoaPods/CocoaPods:
  [Changelog]

* origin/0.10.0:
  remove (unreleased) from 0.10.0 as it has been released

Closes #350

Add support for .tar.gz source file.

Fixed a bug with :head download and no prior clone