Feature/issue 527 normal rounding

#### Summary:
Fixes issue #527 for normal_cdf_log, normal_cdf, normal_ccdf_log.

#### Intended Effect:
normal_cdf: underflow to 0 for scaled_diff < -37.5 and overflow to 1 for scaled_diff > 8.25
normal_cdf_log: underflow to -infty for scaled_diff < -37.5 and overflow to 0 for scaled_diff > 8.25
normal_ccdf_log: overflow to 0 for scaled_diff < -37.5 and underflow to -infty for scaled_diff > 8.25

#### How to Verify:
See unit tests in test/unit/prob/distributions/univariate/continuous/normal.cpp

#### Side Effects:
None

#### Documentation:
See manual under normal distribution.

#### Reviewer Suggestions:
Any.