Implemented CMake build system.

Less setup and maintenance required compared to VS project files, and it
means people can build CBash using a variety of compilers.

`ZLIB_ROOT`, `BOOST_ROOT` and `BOOST_LIBRARYDIR` may need to be defined,
depending on the setup. I tested on my usual setup (self-built binaries)
and the one given on the Building CBash wiki page (pre-built binaries).
Merge pull request #7 from WrinklyNinja/cmake-system

Implemented CMake build system.
A small readme is better than nothing.
Added a bit more text.

I'll add a link to the build instructions once #7 has been resolved.
Added link to build instructions.