Check that calls on external contracts did what we expected

[mmhh1910]

When calling functions in other contracts we make assumptions about what happens there, e.g. with our calls:

Token(token).transfer(msg.sender, amount) Token(token).transferFrom(msg.sender, this, amount)

We should check after these calls, if what we expected actually happened, e.g. after Token(token).transferFrom(msg.sender, this, amount) we expect amount more tokens in our contract than before.