Bugfix/issue 376 phi tails

### Summary

Fix #376 by expanding domain of `stan::math::Phi` as far as is possible (on the non-log scale) using double precision floating point.  

### Intended Effect

* explicitly underflow to 0 for inputs less than -37.5 without calling erfc()
* use erfc() instead of erf() for negative args below -5, 
* explicitly overflow to 0 for inputs greater than 8.25 without calling erf() 
* use erfc() otherwise

No change on derivative behavior.

### Example

Phi(-35) no longer underflows.  See the unit tests for a complete test of the defined range and the agrad tests for tests of the derivative range.

### Side Effects

None.

### Documentation

Added doc for underflow behavior of `Phi` and its derivatives to the manual.

### Reviewer Suggestions

Anyone.  

Does anyone know if there's a better way to test extreme values than `EXPECT_FLOAT_EQ(1, expected / found)` ?