Update README.txt with reference to Erwin's branch and a very basic sample code
Add "external" support to Xcode.

_xcode.lua : based on dev's master version, we disable generating the
file if it's an external (we check for prj.external).
xcode_common.lua : if it's an external, we use the name of the project
instead of the lib's name.
api.lua : based on dev's master version, we add the "external" function.
We disabled checking the UUID since they are different in Xcode
(probably needs some more rework, incremental submit).

Tested on Xcode 4.5.2 only.
Temporarily remove premake4_linux
Update Premake4 binary for Mac OS X 32-bit