Misc: add back nose-timer, support multidimensional variables in traceplot, test discrete step methods

Couple different improvements. I could split this up into different reviews if need be. 

1. Add back nose-timer so we can keep an eye on what takes time to run
2. Support multidimensional variables in traceplot fixing https://github.com/pymc-devs/pymc/issues/512
3. Add a test for discrete Metropolis step method