Introduce IntRect::zero static const var

* Remove get method.
* Rect: Introduce left, top, right & bottom methods.
* Rect: Clean up comments & variable names a bit.
Update comments / documentation
UIWidget: Remove padding methods & class var

These will be replaced by content_margins getters & setter methods
in the future.
UILayout: Clean up alignment code a bit
UIBoxLayout: Add check to not calc bounds when same as passed args
S
S
travis-ci: Fix a few build issues

* Fix tar extraction err when using BSD tar by using single quotes
in the web location passed to wget.

* Fix CMake build warning regarding the missing OSX 10.7 SDK by
providing a local copy for the build to use; the SDK tar ball is
hosted from my DropBox account.

* Fix the installation path for unit test resources when using
XCode project files (generated by CMake); this resolves missing
resources that are needed by a few of the tests.

* Add test failure messages for FontCacheTest::GlobalSystemFontsInterface.

Unfortunately, none of these fixes brings me much closer to resolving
the failing tests -- all I know is that it doesn't like something to do
with BitmapFont objects (TrueType font tests are not failing).