Build on Windows using MSVC, nmake

These commits allow for building on Windows.  I based this on my [ExDoc work](https://github.com/elixir-lang/ex_doc/pull/131) and submitted a PR here per @josevalim's request.  The same steps to build hold:

1. Have Erlang and Elixir on system and in Path variable.
2. Git clone ex_doc into your favorite folder.
3. Install Visual Studio 2013 (Express should suffice, I'm using Ultimate).
4. In a Command Prompt run `"C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\vcvarsall.bat" _ARCH_`, where `_ARCH_` is either `x86` (if you installed 32-bit Erlang) or `amd64` (64-bit Erlang).
5. cd into the ex_doc folder and run `mix compile`.

Requires [elixir-lang/elixir #2426](https://github.com/elixir-lang/elixir/pull/2426).