added CMake support

[CallForSanity]

CMake support might be interesting for many developers.
Tested on Win 8.1 with Visual Studio 2013.

[x42]

The proposed cmake does not explicitly mark relevant functions for export, nor does it support the various configuration options (e.g. built-in hash functions or NSS as alternative to openSSL). It does not run self-tests either, there's no versioning and that's just the tip of the iceberg...

I'm not going to add a 2nd build-system and cmake is not particularly practical for cross-compilation without dedicated handling for it. I'm sorry, this will need further work if this is going to replace autotools.

PS. For the same result as the proposed CMakeLists + MSVC, the easiest is to simply copy the liboauth files into the MSVC project (liboauth is very liberally licensed) and compile+link along with the project.