Remove duplicated code in agrad/rev/matrix/multiply.hpp using enable_if pattern.
Technically agrad::var is not scalar according to boost::is_scalar.  Should probably provide specialization in the future.
Blast, forgot ::value.
Merge pull request #36 from mbrubake/master

New pattern to reduce code duplication in mixed-type agrad functions.
Merge branch 'master' into quad_forms
Start adding more quadratic forms.