Some final cleanups leading up to The Official Switch to Gyp

Some final cleanups leading up to The Official Switch to Gyp
see http://codereview.appspot.com/4580043

1. Create a single public skia.gyp file that all outside projects (Chrome,
Android, etc.) should depend on from now on.  I haven't yet created targets
suitable for those projects to use, but this is where we should add them.
2. Make gyp generate its Makefiles within out/ directory, rather than polluting
directories under source control.
3. Modify trunk/Makefile to automatically run gyp_skia and then call the
generated Makefile, to ease developer transition.

git-svn-id: http://skia.googlecode.com/svn/trunk@1526 2bbb7eff-a529-9590-31e7-b0007b416f81