added: sort and rank functions
Tested version
Added sort and rank to reference manual
Added unit tests
switching between spaces and tabs
Added rev-diff sort versions
Added unit tests for rev-diff sort and some minor changes
Added fwd-diff for sort and its unit tests
Putting spaces between ) and {
Merge branch 'develop' into feature/sort

Conflicts:
	src/stan/model/model_header.hpp
Merge pull request #59 from randommm/feature/sort

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