Feature/issue 458 reject stmt

#### Summary:

Change Stan "raise_exception" statement to "reject" statement.

#### Intended Effect:

Behaviour of "raise_exception" statement remains the same, however the statement name is now "reject" which is shorter and possibly more familiar to Stan's user base.

#### How to Verify:

Behaviour of reject statement is tested in unit/gm tests.

#### Side Effects:

No side effects.

#### Documentation:

Stan reference manual updated - all instances of "raise-exception" including cross-refs changed as needed.

#### Reviewer Suggestions:

no suggestions