Removed the sys dependency

The only use was sys.inherits() which I replaced with the original
node.js implementation.

Preparations for AMD support.
Got rid of `exports`

Created a `HAML` object instead. Wrapped the module inside a define() call.
In the node.js environment, define() is equivalent to module.exports = ...

This adds AMD support.
Added simple test for AMD support

Full test coverage will follow as soon as PR #56 has been merged.
Merge pull request #57 from saschagehlich/feature/amd-support

Feature/amd support