Link the llvm backend to mj.

Add the llvm 3.5 release as a submodule to the llvm directory in the repository root.
	.gitmodules
	llvm

Add a script to build it for integration with mj and add its output to the gitignore file.
	build_llvm.sh
	.gitignore

Building mj is quickly becoming complicated. Outline the steps to get up and running in a README file.
	README.markdown

llvm requres we link with libcurses and libz.
	project/Mac OS X/Compiler.xcodeproj/project.pbxproj

Add link flags for linking the necessary llvm libraries and preprocessor definitions (as stated by llvm-config) and silence a few warnings the llvm codebase does not enforce.
	project/Mac OS X/xcconfig/compiler__common.xcconfig

Creating a dSYM in debug is too slow now that we link llvm. Only create a dSYM for release builds.
	project/Mac OS X/xcconfig/compiler__debug.xcconfig
	project/Mac OS X/xcconfig/compiler__release.xcconfig