Added modulus function
Added integer division function
added modulus rule
modulus working, added unit tests
updated language reference
merging w/ develop to pick up changes to unit tests for gm/parser
made int division behaviour consistent w/ modulus, modified unit tests, unit tests for parser split into groups of related tests
modified per pull request comments
added/renamed unit test file
more division tests
function int_divide now called divide, warning message for int division reinstated, unit tests updated accordingly
Update and rename int_divide_test.cpp to divide_test.cpp
Merge pull request #820 from stan-dev/feature/issue-577-modulus-operation

Fixes #577. Feature/issue 577 modulus operation