Don't return non-contiguous rounds

[iFrostizz]

See #64 (comment).
Events are allowed to be imported out of order. The issue is that the assumption is different from the original Chainlink contracts.
We should still store them even if the rounds are not contiguous, it's just that they should not be returned from the getRoundData function.
It is actually slightly harder than thought first and will leave it for later. Here is one solution though:

Write a library that stores 256 bits maps, and everytime a new round is added, add it to the bitmap and check if there is any missing round. If no, then store the latest roundID as a validRoundID.
In the getRoundData, make sure that the requested roundID is less than or equal the validRoundId.