WIP New installer.

View the PR diff at this URL to avoid seeing whitespace changes: https://github.com/nodegit/nodegit/pull/140/files?w=1

- Converted to Promises.
- Providing support for Windows.
- Normalize paths correctly based on platform.

Currently failing on Windows.  I've commented out two offending casting lines that will need to be addressed later.  Lets get this thing build and linked first!

I need help with the binding.gyp file.  I've tried many combinations and it currently does build, just link errors all over the place for every libgit2 external symbol.  Definitely doing something wrong.

If you want to help fix this:
(Ensure you have cmake and python2 installed first)

``` bash
# Clone the repository.
git clone --branch=new-installer https://github.com/nodegit/nodegit.git
cd nodegit

# Install dependencies.
npm install

# View link errors with:
node_modules\.bin\node-gyp configure build
```