Feature/clean up tuplegetter

* This makes TupleGetter/Setter/Converter follow the type-class pattern.
* This significantly cleans up the implicits we were importing into Job, as they are no longer needed.
* This declutters the docs on Job and other types that were including TupleConversions (which has been changed to an object rather than a trait).
* This is a breaking change that should be a candidate for 0.9.0.

On a personal note, this changes some of the first significant code I added to scalding about 1.5 years ago:

https://twitter.com/posco/statuses/106477842052825088