models: start adding framework for parameter derivatives.
Merge branch 'master' into param_derivs
Merge branch 'master' into param_derivs

Conflicts:
	pint/models/timing_model.py
Finish merging parameter changes.
Merge branch 'param_derivs'

Conflicts:
	pint/models/timing_model.py
Merge branch 'anne_orbits'

Conflicts:
	pint/models/parameter.py
Remove old test_resids.py
Fix a few comma-space lint warnings
Adding parameters now looks just like assignment
Get the help printing function working and tested
Time dilation.
Fix glitch
pylint cleanup
Merge branch 'orbits' of github.com:aarchiba/PINT into orbits

Conflicts:
	pint/orbital/kepler.py