Feature/issue 440 log prob for eigen

#### Summary

Add a log_prob function to generated models accepting Eigen vector argument for reals and dropping int

#### Intended Effect

Making the above log_prob function available.

#### Side Effects

None --- the old function with int vector is still there

#### How to Verify

Look at unit test or output of model compilation.


#### Doc

Not user facing.  Should update Wiki on model concept after it gets into a Stan release.


#### Reviewer

Anyone.  It's a trivial update (though writing the tests involved using the makefile fiddle Daniel created, which leaves a less-than-optimal hard-coded dependency for the test in the makefile for unit tests


* [x] code review