Non-void functions can omit return expressions

[DanielRosenwasser]

The old compiler was permissive with both of the following code snippets:

function foo(): number {
    return;
}

class C {
    public foo() {
        return;
    }
}

as per the 1.0 spec (6.3):

An explicitly typed function whose return type isn't the Void or the Any type must have at least one return statement somewhere in its body. An exception to this rule is if the function implementation consists of a single throw statement.

I believe we should change this to

An explicitly typed function whose return type isn't the Void or the Any type must have at least one return expression somewhere in its body. An exception to this rule is if the function implementation consists of a single throw statement.

It seems weird that we allow implicit return of undefined through return statements with no explicit expression, but we don't allow the implicit return of undefined through by omitting return statements entirely.

[RyanCavanaugh]

Forward-duping to #228