Initial split of optimizers/statistics
PEP 8 and docs changes
add example of a user-defined statistic function
Add changelog entries for the changes introduced by this PR
Fix up the examples a bit to reflect the current code.  I agree with @taldcroft that these examples expose an unfortunate degree of complexity involved in creating new Fitters, such that it is not really at a point where casual users can do it.  But these docs are still useful to have for developers to look at--in the future I hope to try to hide more of the sausage-making if I can.  Also redid the section headings per the preferred formatting.
Remove this no longer relevant comment
Fix typo
Fix a few more references to NonLinearLSQFitter
Just a little test module cleanup
A little more cleanup.  Use the assert_ function directly instead of going through a 'utils' module.  Use the NumpyRNGContext where appropriate.
As requested, adds a simple test for the SimplexLSQ fitter.
A slew of minor documentation fixes--many of them for things pointed out by @cdeil in #2441, but also including several other mistakes I found while reading through the docs.
Add an electron unit

This is used heavily in ccdproc; having it in core astropy would be convenient
Add formatting to electron unit
Add changelog entry
Attempted fix of #2597.  Forcibly put the "new" astropy at the front of
the sys.path when building wcs test extension.
Fittable models should not be linear by default, though I added linear=True to a couple models that are clearly linear in their parameters
Fix typo in a couple of docstrings
Followup to 121eb3ff5d8d2dad72993ad36d2e27e1683a8148 -- the header argument also was not being passed through.
Merge pull request #2599 from mwcraig/add-electron-unit

Add an electron unit