Added documentation, improved expdesign.

intro.rst improved with content from README.rst. ExperimentDesigner
now supports callable guesses and multiple-field expparams_dtype.
Merge branch 'master' of github.com:csferrie/python-qinfer
Added better heuristic support, started Heuristic class stub.

Heuristic functions can now accept "idx_exp", "mean" and "cov"
keyword arguments. An empty class has been added for future
improvements to heuristics.