Model addition, predefined subclass Models, and lots of unit tests

Wheeeee....

Predefined ``Model`` subclasses in ``lmfit.models``:

    'Constant',
    'Linear',
    'Parabolic',
    'Quadratic',  # synonym for Parabolic
    'Polynomial',
    'Exponential',
    'Gaussian',
    'NormalizedGaussian',
    'PowerLaw'

Model addition:

    >>> model = Model(user_defined_func, independent_var=['x']) + Model(another_user_func, ['x'])

    >>> model = Gaussian(['x']) + Constant()

    >>> model = PowerLaw(['x'] + Polynomial(4, ['x'])  # specify any order polynomial

    >>> model = Gaussian(['x']) + Gaussian(['x'])  # raises error because parameter names conflict

    >>> model = Gaussian(['x']) + Gaussian(['x'], suffix='2')  # works fine

There are unit tests for all.

TODO:

* Automatic "guessing" as in ``models1d.py``. This will require some messing with the API...currently, Model raises if the user fails to specify any parameters.

* More models.

closes #63