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.