Temporarily disable failing test (issue #70)
Mark some failing tests as xfail (GH issues created).
Add empty FluxPoint class to make old tests work.
Move gammapy/spectrum/tests/test_data.py -> gammapy/spectrum/tests/test_flux_point.py
Mark tests xfail instead of commenting them out or adding '_'.
Add `spectrum.flux_point` to docs
Disable sphinx build on travis-ci until we switch to wheels.
Add link to introduction.
Add XSPEC effective area functions.

Adapted from PyFACT.
Add energy dispersion code (XSPEC RMF style).

I/O code from PyFACT, the rest I wrote myself.
Add xspec_fake.py and xspec.fits examples (unfinished)
Add effective_area and energy_dispersion to top-level irf namespace
PEP8 change.
Rename function in effective area test
Fix typos in effective_area and energy_dispersion IRF code.
Make test using uncertainties optional
Add `docs/credit.rst` page
Include np_to_pha function (modified) from PyFACT
Add image_to_hdu utility function from PyFACT
Remove `log_mean_energy` function ... use `scipy.stats.gmean` instead.