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.
Use the safe tempfile.NamedTemporaryFile in SExtractor unit test.
Add unit test for np_to_pha function.
Move image_to_hdu to pyfact.py
Temporarily add PyFACT stuff in gammpy/utils/pyfact.py
Add some entries to docs/references.rst
chmod scripts to be executable
Move over `sim_evlist` and `create_spectrum` from PyFACT