Linux support

Hi,

This pull request contains (experimental) support for Linux. Tested on Ubuntu 14.04 64-bit.

The pull request is a bit largish, with all kinds of patches that I encountered while making CppSharp work on Linux. Only some of them are strictly related to making CppSharp work on Linux. If you want, I can split these into smaller pull requests, but I thought I'd first try with one big pull requests.

Some notes:
- I've updated the build instructions related to the LLVM install, and to describe how I built CppSharp on fresh Ubuntu install.
- LLVM needs to be installed to deps/llvm/install, instead of being used from the build dir
- CppParser bindings for Linux (generated on Windows) are included.
- I have added deps/x86_64-linux-gnu.tar.xz, which contains the headers needed to create Linux CppParser bindings on Windows. I don't like binary blobs in the git tree, but I though this might be the easiest way to let people rebuild the Linux bindings.
- I have added build/premake5-linux, for the same reason as above.
- Only simple cases work on Linux so far. The tests included in CppParser fail.