Add Panics()

[elliotchance]

This works the same way as Errors in #1:

Divide(5, 0).Panics() // pass
Divide(5, 0).Panics("divide-by-zero") // pass
Divide(5, 0).Panics("foo") // fail

[m1ome]

I am on it, also some refactoring required to make it done.