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
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 #686 from stan-dev/feature/issue-527-normal-rounding

Fixes #527. Feature/issue 527 normal rounding