Max Attempts

This limits the number of POST attempts for a given object to `client.maxAttempts`, which defaults to 3. After `maxAttempt` attempts, the event will be deleted from the db.

I also considered leaving it in the database, but not attempting to POST it again, but I could think of a situation in which that would actually be useful. Maybe if there was a bug in the SDK and then we patched it, old events that were stashed away could be attempted again?

#72 needs to be finished and merged before we merge this so once that is in I will rebase. In the meantime, you can see just these changes by looking at [this comparison](https://github.com/smurthas/KeenClient-iOS/compare/smurthas:retry-count...smurthas:max-attempts).