Enable multi_gp in the language.
Add multi_gp to the manual.
Fix inverse in manual.
Fix function signature.
Add agrad unit tests for multi_gp_log.
Merge branch 'feature/add-multi_gp' of git://github.com/mbrubake/stan into feature/issue-359-add-multi-gp
Merge branch 'develop' into feature/issue-350-add-multi-gp
Merge pull request #2 from stan-dev/feature/issue-350-add-multi-gp

Feature/issue 350 add multi gp
Move multi_gp gradient test to new folder
Merge pull request #3 from randommm/feature/multi-gp

Feature/multi gp
Merge pull request #350 from mbrubake/feature/add-multi_gp

Fixes #350. Re-add multi_gp distribution