Feature/issue 592 expose isnan isinf

#### Summary:

Adds functions is_inf and is_nan to modeling language.

#### Intended Effect:

Functions is_inf and is_nan can be used in tranformed data and transformed parameters sections to detect numerical problems arising from operations on the data and parameters.

#### How to Verify:

Run unit tests for functions in stan/math/functions, stan/agrad/fwd/functions, and stan/agrad/rev/functions.  Run test test/integration/compile-models to compile Stan models which use these functions.

#### Side Effects:

None.

#### Documentation:

Added (minimal) documentation to the Stan reference manual for these functions.

#### Reviewer Suggestions:

no suggestions