optimizing() method working

Need to spend some time figuring out the output format for the optmizer.
Pystan returns a simple dict, and doesn't use the StanFit object. Seems
more sensible to reuse the StanFit object if possible. Might be worthwhile
to subclass something abstract, but may be overengineering?
Various cleanups to get optimizing integrated

stan()
 - sets defaults for iter and warmup like Pystan
 - accepts 'fit' to avoid recompilation
 - 'method' input controls whether optimizing or sampling
 - documentation

StanModel
 - set() methods handles most parameters
 - algorithm can be set, depends on method (sample/optim)
 - set.refresh uses validator
 - if .stan file already exists, a check is implemented to avoid
   touching file if contents are identical

StanFit
 - functions for both optimizing and sampling
 - verbose attribute, can be set dynamically
 - moved static methods to +mstan