Feature/issue 577 modulus operation

#### Summary:

Add binary infix modulus operator for integer operands to Stan language.

#### Intended Effect:

Users writing Stan models can do modulo arithmetic via infix operator "%". 

#### How to Verify:

Added tests to unit test test/unit/gm/parser_test which attempts to compiles two models:  src/test/test-models/syntax-only/validate_modulus_good.stan and src/test/test-models/reference/validate_modulus_bad.stan. The former compiles, the latter generates a Stan compiler error.

#### Side Effects:

none?

#### Documentation:

Added documentation to the Stan reference manual, language section, subsection on expressions.

#### Reviewer Suggestions:

Parser geeks.