Adds AssignableTo matcher

This pull request adds a matcher to match the type of two variables. This is helpful in scenario's where you do not interested in the value of something, but in it's type.

Here is a simple assertions that demonstrates its usage:

``` go
Ω(5).Should(FitTypeOf(-1))        // different values same type
```

Currently the implementation is based on `reflect.Type.AssignableTo` method. In the future we could add more scenario's based on the `Expected` type, like checking if something implements an interface or not.