Pygtc

* It is now possible to omit a parameter from a chain by using None as a
placeholder. Demo notebook and documentation updated appropriately.
* Added changelog to manifest.

* No longer fails if you ask for priors and scipy is missing. Instead, it
issues a warning and ignores the request.
* Test suite now skips tests that required pandas or scipy if pandas or scipy
is missing.

* Refactored code to make contour choice options more clear.
* Updated documentation with discussion about contour level choices.
* Added tests for most Keyword Argument options. Tests require nose.
* Updated documentation to include notes about testing.
* Added a requirements.txt file to the package specifying version numbers and
removed those specifications from setup.py.

* Option to turn off tick label rotation
* Long tick labels are now lined up correctly with their ticks
* Option to fine tune position of label relative to tick
* Can choose to use 1d or 2d Gaussian sigma contours
* scipy is now an optional dependency
* Can choose to fill contours and 1d histograms independently
* Can plot the actual data-point density underneath the contours
* Full control over fonts for labels, ticks, and legend
* Use built-in matplotlib LaTex renderer for equations in labels so no LaTex
installation is required locally.
* Added a new example to the documentation

* Prettier truth colors
* Parameter labels line up even if parameter numbers are very different in size
* Fixed bug where parameter labels near the edge could overlap

* Initial version