Feature/issue 856 int step nan

#### Summary:

Adds test for `stan::math::int_step`. Adds documentation to C++ function for NaN input.

#### Intended Effect:

No change to behavior. Only adds tests and doc.

#### How to Verify:

Run unit test in src/test/math/functions/int_step_test.cpp.

#### Side Effects:

None. No change to behavior.

#### Documentation:

Documentation for C++ function has been changed. 

#### Reviewer Suggestions:

Anyone.