fixes issue #464
added diag_post_multiply to signatures and tested compilability
added tests for pre/post multiply, fixed bug in post multiply
added pre/post-multiply tests for agrad matrix rev
Merge branch 'develop' into bugfix/issue-464-diag_pre_multiply
moving test files
updating unit-agrad-fwd/matrix/diag_post_multiply test
Merge pull request #523 from stan-dev/bugfix/issue-464-diag_pre_multiply

Fixes #464.