ENH: Better separation of StanFit and underlying samples

Created a class mcmc for storing samples. Allows for simpler StanFit
object, and it will be easier to extend mcmc to do some of the work
that coda in R does.

Squashed commit of the following:

commit 039b62ec49532400ac8b0ff4fec3e6e8d5fcdf3b
Author: Brian Lau <brian.lau@upmc.fr>
Date:   Fri Jan 24 09:08:22 2014 +0100

    move traceplot to mcmc object

commit 9d3c366ee6007de9fa6af8eb883e569eb5cbc7d2
Author: Brian Lau <brian.lau@upmc.fr>
Date:   Fri Jan 24 08:53:05 2014 +0100

    update tests for mcmc object

commit 323352a6201205c2b49d1b9c16b424744542fefe
Author: Brian Lau <brian.lau@upmc.fr>
Date:   Fri Jan 24 08:08:37 2014 +0100

    moving some StanFit methods to mcmc

commit 6d966036b4c5a82529ff7d7991ae617a8a14c473
Author: Brian Lau <brian.lau@upmc.fr>
Date:   Thu Jan 23 21:50:06 2014 +0100

    initial commit of mcmc object
Cleaning up testings
Clean up mcmc tests
BF: default to reading file from pwd if path missing
ENH: workaround for pre-2012a Matlab
BF: missing validators for some hmc params
ENH: Clean up unit testing, expose remaining Stan params

Units tests run clean on OSX, Matlab 2011b, 2012a/b

StanModel()
 - Add control parameters (adapt & hmc relevant for sampling)
 - select_file() method, brings up gui for selecting stan file
 - delete_binary() method, removes C++ and associated binaries

StanFit()
 - move more stuff to mcmc class