Change 'LastUpgradeCheck' for Xcode 4.5

Changes 'LastUpgradeCheck' from '0420' to '0450'. This suppresses the
upgrade warning in Xcode 4.5.
Add basic support for Resources Build Phase.
Typo fix
Merge branch 'support_for_resources_build_phase'
Add some spec for PBXResourcesBuildPhase
Merge pull request #30 from pbernery/support_for_resources_build_phase

Support for resources build phase
WIP: cache result of @callbacks[:uuid_scope] in PBXObjectList and update @uuid_scope when object is added/removed
[NativeTarget] Support .hh files as headers
Merge pull request #32 from jonathanpenn/master

Support .hh files as headers
Merge pull request #28 from claybridges/Xcode45-upgrade-check

Change 'LastUpgradeCheck' for Xcode 4.5
[Config] Small code cleanup.
[Fixtures] Added sample project.
[Bundle/Yard] Update and use of flawored markdown for documentation.
[Gitignore] Added xcuserdata.
[Fixtures] Updated sample project.
[Project/Object/ObjectList] Refactor.

The callback architecture was dropped in favor of reference counting. With this
refactor Xcodeproj builds a tree of objects and then converts the tree to hash,
instead of using a hash for manipulating the project.

Other changes:

- `ObjectList` is now an `Array` subclass.
- Runtime type checking for attributes.
- Caching of UUIDs.
- Other API changes.

Related to #6, #7, #26, #31.
[Constants] Added COPY_FILES_BUILD_PHASE_DESTINATIONS.
[Workspace] Commented source code.
Removed xcuserdata from sample project.
[Hash] Comment update and added spec.