First stab at revising Metropolis. Can now specify alternative proposals. Still need to add tuning.
Added tuning function for step functions
Re-implemented tuning to keep logic in step function
Fixed proposal functions for Metropolis
Moved tests into pymc/ and made them easily runnable as a batch
Reverted matplotlib backend change in test_examples
Merge pull request #214 from pymc-devs/metropolis

Add self-tuning to metropolis step method.