massive overhaul for updating syntax to allow 'with model:' and x = Normal('x', ...)

Conflicts:

	examples/tutorial.ipynb
forgot a file
refactored model contexts adding implicit context to find_MAP, approx_hess, sample etc. Added deterministic traces
made NpTrace slightly more DataFrame like for forwards compatibility
added arbitrary stochastic dist
add arbitrary potentials
fixed examples and added deterministic traces
fixed up tests
added laplace distribution
remove psample
fix error checking on get_context
fixed metropolis scaling
added 2d kde plot and made kde available as stand alone plot
added block updating example
gotta run in tests
fix plot
fix Metrop test
added progress bar
javascript progress bar
added deterministic traces