WIP: Add more power law models

I'm adding a few more power law type models that are commonly used in spectral analysis (see e.g. [here](http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/Cicerone/Cicerone_Likelihood/Model_Selection.html))

As a first step I've put the existing `PowerLaw1DModel` in it's own file and added an `x_0` parameter to it.
It's useful to have `x_0` as a parameter because by setting it one can control the parameter correlation between the amplitude and the spectral index.
In a fit `x_0` should usually be fixed though, so maybe we should set it to a default value of 1 and set it fixed in the constructor?

@adonath, @nden, ... what do you think?