Added (optional) messages to Assertions

[theneubeck]

I was missing the possibility to add a custom error message to an assertion. So I added it:

g.Assert("My string").Equal("Other string", "'Other' is not equal to 'My'")

And more useful:

g.Assert(user.IsValid()).IsTrue("Expected user to be valid")

plus

g.Assert(user.IsValid()).IsFalse("Expected user to be invalid")

[marcosnils]

@theneubeck thanks for contributing to goblin.

As a first basis I see that travis complained about tests not passing. Can you please review them and send the PR again?

As a second comment and per your examples I don't see how specifying a message could be useful in the Equal case. Because the equal assertion already prints the differences between the supplied arguments, so I your example I see that the message is somehow redundant. Can you please explain if you have some other cases in mind that specifying a message for Equal might be useful?

Regarding to adding a message to the IsTrue and IsFalse assertions I believe that it's a good idea as Goblin output is not very friendly when it comes to booleans.

[theneubeck]

Hi, sorry for the broken build (it doesn't seem to fail with my version of go) I'll review it.

Regarding the feature. It true that it was in the IsTrue/IsFalse case I felt the need to add a message. But since it seems pretty standard to be able to giva a custom message to assertEqual, I figured that I should add it, while at it. For example java's junit (http://junit.sourceforge.net/javadoc/org/junit/Assert.html) and python (https://docs.python.org/2/library/unittest.html) has that feature.

If you'd rather I remove the custom message from Equal and Eql, I can change it.

[marcosnils]

Let's stick with the message for the IsTrue and IsFalse for the moment. Whenever there's a real use case to add it to the Eql and Equal assertions we'll add it.

Please, go ahead and make the changes so we can review!.

[marcosnils]

I didn't see that you already made the change. Merging!.