Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
added switch to math::Phi to use erfc to extend negative domain, no change in derivative code
added tests for derivative overflow/underflow and doc about overflow/underflow to manual.  completes fix of #376
Merge pull request #430 from stan-dev/bugfix/issue-376-Phi-tails

Fixes #376.