Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-648-pow-infix-syntax
added infix exponent operator, unit test
added file for exponent operator unit test
test exponent operator throws exception
added exponent operator description to user manual
cleanup comments
Merge pull request #681 from stan-dev/feature/issue-648-pow-infix-syntax

Fixes #648. Feature/issue 648 pow infix syntax