sort_asc(v), sort_desc(v) and rank(v,s) functions

Added functions sort_asc(v), sort_desc(v) and rank(v,s).

They were tested using this model file:
https://gist.github.com/randommm/5512981 for both RStan and command-line.

I didn't found a good section to put them in the reference manual, so I created a subsection Other Function for both Array Operations and Matrix Operations. But I can change this if you wish, just tell me what would be the subsections where those functions would fit better.