fixed rounding error for normal_cdf, normal_cdf_log with tests
updated test names
added ccdf_log tail tests
fixed test name
added manual doc about behavior of normal_cdf functions
support scalars in elementwise division (./)

fixes #656 by adding support for
elementwise division of a scalar by vector/matrix
and elementwise division of a vector/matrix by scalar
Merge branch 'develop' into feature/issue-527-normal-rounding
moved *SQRT2 om LHS to *INV_SQRT_2 on RHS
Merge branch 'develop' into feature/issue-527-normal-rounding
Merge pull request #660 from alyst/elt_ops_extend

Support scalars in elementwise products
Merge pull request #686 from stan-dev/feature/issue-527-normal-rounding

Fixes #527. Feature/issue 527 normal rounding
removed stan::agrad in Phi_approx
Merge branch 'develop' into feature/issue-675-check-autodiff