implements avrt

[k-brac]

Implements avrt
Adds a macro for enum having negative values

[retep998]

With the -gnu targets Rust is able to work standalone without a separate C/C++ toolchain. This means that the only import libraries which are available are either those that Rust bundles, or winapi bundles. In the case of avrt, Rust does not bundle it, therefore winapi must bundle libavrt.a itself. The current way of resolving this is getting libavrt.a from a mingw-w64 installation and putting it in the correct location. See https://github.com/retep998/winapi-rs/tree/master/lib/secur32 for an example of this. However in the near future I will likely be solving this with #302 instead.

[retep998]

Strangely enough, AVRT_NOTIFICATION was actually removed in the Windows 10 SDK. Since I try to track the latest Windows 10 SDK with winapi, I'd rather this not be here, unless you can provide a compelling reason for why it should be kept.

[k-brac]

My bad. My visual studio was not properly setup to use the latest sdk... Hence I removed AVRT_NOTIFICATION
The rest doesn't seem to have been modified

[retep998]

This PR will need to be updated for the winapi 0.3 new world order. #316

[k-brac]

merged in #348