Fix for false imputation issue in #529
Merge pull request #530 from pymc-devs/impute_fix

Tested clean. Merging.
Added Slice sampler to 2.3 code base
Incremented version number to 2.3.3
Edited definitions of chbevl to fix #549
Tweak to .travis.yml to fix MPL display problem; fixed some math in the docs
Changed MathJax to use https server
Tweaked Slicer to support vector-valued stochastics
Added utility function for calculating posterior predictive p-values
Renamed post_pred_check to ppp_value and moved it to the diagnostics module
Greatly simplified ppp_value
Fixes to slice sampler
Added Slice sampler to 2.3 code base
Incremented version number to 2.3.3
Tweaked Slicer to support vector-valued stochastics
Fixes to slice sampler
Merge branch '2.3_slice' of github.com:pymc-devs/pymc into 2.3_slice
Ensured all tests passing
PEP8 compliance changes
Added documentation for the Slicer step method