Port AMX Mod X to AMBuild.

This branch removes the old C# build tool and all of its versioning gunk and introduces AMBuild 2 scripts. In general this was pretty straightforward, the new versioning isn't ideal but I didn't want to get too invasive. The precompiled binaries in scripting were also removed since AMBuild makes it easy to build them.

I've tested a fair bit of this, it builds on all 3 platforms and binaries still load on Linux. More testing will definitely be needed, but I'd like to wait until buildbot hooks are up.

The new package names look like:
`amxmodx-1.8.3-dev-git3749-base-linux.tar.gz`