generalized optional dimensions to sequence of dimensions to allow double-indexing on LHS of assignment
test for new statement grammar indexing
more throrough assignment indexing test
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
models updated, doc updated, semicolon added to end of increment_log_prob function
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/increment_log_prob
Merge pull request #163 from stan-dev/feature/issue-159

Fixes #159.
Merge branch 'develop' into feature/increment_log_prob

Conflicts:
	src/stan/gm/ast_def.cpp
Merge pull request #171 from stan-dev/feature/increment_log_prob

Feature/increment log prob