Merging into develop

Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merging branch
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Efficient implementation of multiply_lower_tri_self_transpose.hpp
Modified test file to include symmetry checks on large matrices
Merge branch 'develop' into bugfix/issue-1121-multivariate-error
updated test
Merging
Merge branch 'bugfix/issue-1121-multivariate-error' of https://github.com/stan-dev/stan into bugfix/issue-1121-multivariate-error
Removed old method from unit/math/matrix test, changed the large L matrix generator function, added the test to agrad/rev/matrix test, cleaned up commented code
Merge pull request #1123 from stan-dev/bugfix/issue-1121-multivariate-error

Fixes #1121. Bugfix/issue 1121 multivariate error