Develop

I rewrote most of the mode; most of the user facing stuff is the same, but a lot has changed in the background. @syclik, do you want to look it over or try it out before I merge this into master? 

The major changes are:

- updated the keywords and built-in functions to Stan 2.0.1. The biggest change is that the function names are now taken directly from the reference manual.  The code in the directory `keywords` parses a text version of the reference manual and creates a json file `stan_lang.json` which contains keywords, and the function signatures from the manual. This is used to create the file `stan-keyword-lists.el` which contains the lists of keywords and built-in functions used by `stan-mode`.  Having a file with all the function signatures will also make it easier to add auto-complete and similar functionality to the mode.
- no longer derived from `c++-mode`, I added it as a new `cc-mode` language.  This allows it to use `cc-mode` indentation engine, but without any unnecessary baggage from `c++-mode`.
- `stan-mode`, `flymake-stan` and `stan-snippets` will be split out as separate packages on MELPA. This makes dependency tracking easier.