Flux point method

Implements #50 .
I'm not sure if this is useful ... can't the same thing be done e.g. with 
[numpy.polyfit](http://docs.scipy.org/doc/numpy/reference/generated/numpy.polyfit.html) or [numpy.linalg.leastsq](http://docs.scipy.org/doc/numpy/reference/generated/numpy.linalg.lstsq.html)?

If this actually is something that can't easily be done with numpy or scipy, then OK to put it in gammapy, but please somewhere in `gammapy/utils` instead of `gammapy/spectrum/utils.py` because it's not spectrum-specific.