fixed up tutorials
Added clarifying comments to the tutorial.
Minor edits to notebook.
Raise error for sparse matrix without scikits.sparse installed in hmc
Reverted HMC
Merge pull request #224 from johnmcdonnell/tutorial-fixup

Added clarifying comments to the tutorial.
updating python versions
Merge pull request #215 from pymc-devs/tutorial-fixup

Fixed up IPython Notebook tutorials to explain new syntax