Flux point method

Implements #50 .
The first thing I tried was this and it failed with a weird error message:
```
>>> from gammapy.spectrum import compute_differential_flux_points
>>> from astropy.table import Table
>>> table = Table(dict(ENERGY_MIN=[1,2,3], ENERGY_MAX=[2, 3, 4], INT_FLUX=[43, 44, 54]))
>>> compute_differential_flux_points(table)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/deil/code/gammapy/gammapy/spectrum/flux_point.py", line 47, in compute_differential_flux_points
    g = np.abs(spec_index)
TypeError: bad operand type for abs(): 'NoneType'
```

Make a global variable `DEFAULT_SPECTRAL_INDEX = 2.` and use it here.