Merge pull request #136 from newville/master

update docs (not quite done) for 0.8.0
need warnings imported, simplified fitter warnings
py2.6 compatibility (why not?)
fix typo with weights
add test(s) to verify that weighting alters results
more doc improvements
add example of using per-iteration callback
improved doc for confidence_interval trace
add skewed Gaussian function and model
add skewed Gaussian function and model
doc tweaks to copyright notice
doc: simplified words in top level doc (index), moved some to parameters
add authors to license
0.8.0
doc typo
updated README, using rst formatting
updated README, using rst formatting
add Zenodo DOI badge
add fit_kws argument to fit method in Model
clean up files