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")
```


I believe that this test is not necessary as IsTrue and IsFasle are already tested in the assertion_test file. 

We can go ahead and remove this test as it confuses more than what it describes. After that PR will be ready to merge.

We really appreciate your contribution!.