Added (optional) messages to Assertions

I was missing the possibility  to add a custom error message to an assertion. So I added it:

```go
g.Assert("My string").Equal("Other string", "'Other' is not equal to 'My'")
```

And more useful:

```go
g.Assert(user.IsValid()).IsTrue("Expected user to be valid")
```
plus

```go
g.Assert(user.IsValid()).IsFalse("Expected user to be invalid")
```