added validate equal in similar fashion to validade_greater, validate_greater_or_equal, etc
Added function matrix scale(matrix, vector), relevant unit tests and documentation
Some small improvements to diag_pre_multiply and diag_post_multiply
Applying changes recommended by Marcus Brubaker
Renamed function scale to quad_form_diag
Group quad_form_diag with quad_form in the manual
Merge pull request #606 from stan-dev/feature/fix-issue-244-scale

Fixes #244. Feature/fix issue 244 scale