Added GPU model example, basic performance testing.
added __all__ to test_models.
Added MASMC to smc.py's __all__.
More fixes to make simple example work, new ExpSpareHeuristic.
Fixed doc errors, added details of new classes, functions.
Added more documentation, fixed dists, fixed doc issues.
Added new check to resamplers.py for zero-norm cov.

I noted in testing that sometimes an updater would get very
close to the true model, but would fail with a ResamplerError
due to the estimated norm of the error in sqrtm(cov) being nan.
This can happen either if the error term is awful, or if the
norm of cov is zero; the error is intended for the former case,
not the latter. To compensate, the LW resampler as of this commit
now checks for zero-norm cov and if detected, warns, then adds
a small artificial covariance to compensate.
Added logic to clear_cache to prevent IPython parallel memory leaks.
Merge pull request #15 from csferrie/feature-perftesting

GPU models, performance testing.