Feature/issue 257 abs deprecate

I think this fixes the issues with absolute.  I deprecated the use of abs(real) in the modeling language, which really just means it prints a warning.  

I added a ```stan::math::abs(double)``` which just delegates to ```std::fabs(double)```.  The behavior was broken in Stan 1.3 because it called ```std::abs(int)``` and did the downcast.  

I made sure by running some models that it did the right thing, but I didn't include model tests.  The model syntax is already being tested with the function.