Add Rugged cross compilation support

This adds support for cross compilation of rugged binaries for win32. It depends on the `Makefile.embed` changes I proposed in libgit2/libgit2#1748.

#### How To Use

In general, follow the steps as described in the [rake-compiler readme](https://github.com/luislavena/rake-compiler#how-i-do-this-from-linux-or-osx) to install the mingw-w64 toolchain, add it to your path and cross-compile one or multiple ruby versions. Ruby version `2.0.0-p247` worked for me.

Next, run: `bin/rake cross native gem`

And a few minutes later, you should have a shiny new binary gem for win32. :metal: 

```
  Successfully built RubyGem
  Name: rugged
  Version: 0.19.0
  File: rugged-0.19.0-x86-mingw32.gem
```

#### Building "Fat" Gems

Building a "fat" gem for multiple ruby versions for windows is easy, too:

Make sure you have cross compiled a ruby 1.9.3 and a 2.0.0 version on your machine, then run:

```
bin/rake cross native gem RUBY_CC_VERSION=1.9.3:2.0.0
```

#### Still To Be Done

* [x] Fat gem support, so that we can have one gem for 1.9 and 2.0 users.
* [x] Fix unit tests on windows. Most of them fail due to the issues fixed in #237.