[Workspace] Support file reference types

The following types are supported by Xcode:

- Absolute Path (absolute)
- Relative to Group (group)
- Relative to Workspace (container)
- Relative to Developer Directory (developer)
Merge branch 'master' into file_reference_type

* master: (38 commits)
  Release 0.16.1
  Fixed issue #147
  Support .ipp files as headers
  Release 0.16.0
  [Constants] Disable errors on warnings for default build settings
  Bump version to 0.15.3.
  [rake] Fix the gem buid and release tasks to include prebuilt binaries.
  Bump version to 0.15.2
  [CHANGELOG] Prepare for 0.15.2 release.
  Fix prebuilding on OS X 10.9's Ruby.
  Revert "[Ext] Remove -multiply_definedsuppress argument from prebuilt makefiles"
  Fix Apple's breakage to Ruby's build config.
  Revert "[Ext] Fix -multiply_definedsuppress argument for system Ruby"
  Release 0.15.1
  [Rakefile] Add title to build task
  [Ext] Fix -multiply_definedsuppress argument for system Ruby
  [Ext] Remove -multiply_definedsuppress argument from prebuilt makefiles
  [Rakefile] Adopt Rainforest & streamline
  Revert "Remove empty OTHER_LDFLAGS setting"
  Update README.md
  ...
[Workspace] Use dedicated file for FileReferences class
[Workspace] Minor editing to FileReference class
[Workspace] Minor editing to FileReference class 2
[Workspace::FileReference] Fix equality comparison
Merge pull request #150 from CocoaPods/file_reference_type

[Workspace] Support file reference types