ENH: Add Model class and example.

This is an attempt to implement the ``Model`` API shared by @vnmanoharan in #56. This implementation also allows an even more concise usage, generating Parameters implicitly from keyword arguments. View [this IPython notebook](http://nbviewer.ipython.org/github/danielballan/lmfit-py/blob/4d4e0f1115bad70f7ad900f9da27d9d99ea996c9/examples/lmfit-model.ipynb) for some hasty documentation.

Thoughts?