Exception message

[electricjones]

Referring to #16, this PR makes it possible to optionally test the exception message.

Of course, this is only an enhancement, the current api has not changed.

$this->specify('user is invalid', function() {
            throw new RuntimeException("test message");
        }, ['throws' => ["RuntimeException()", "test message"]]);

A couple notes

• Works for all kinds of exceptions
• Works for all kinds of throws parameters (object, string, fail(), etc)
• Is an invisible change, meaning the user does not have to check for message
• Passes all tests, adds new tests, and updates documentation.
• Only added a few lines of code

I am willing to mix this up if needed. We needed this for our team and just thought I'd share it with the world.

[DavertMik]

Thanks. I'll release a new version for that

[electricjones]

Appreciate it. If you need anything else, let me know.