Building ex_doc on Windows with MSVC

These commits allow for a successful build on Windows using Visual Studio's MSVC distribution (bar one snag, detailed later) by following these steps:

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`.

Currently, (5) will throw a "permission denied" error.  For now, just run `mix compile` again and all will go well.  Similarly run `mix test` twice, and all tests should pass.

Closes [#44](https://github.com/elixir-lang/ex_doc/issues/44)
Can't you make something like the following:

1. Check if the `Makefile.mix.win32` is available
2. If not, you generate one, in the Makefile itself and call `exec make priv\markdown.dll`

I am not sure `exec` would work on windows but windows must have an equivalent, no? It basically replaces the current process by the one that will run the given command.