`onBackpressureBuffer(int)` errors on overflow

[abersnaze]

Talking with @neerajrj this morning about various Rx issues. One of them is that onBackpressureBuffer(int) defaults to sending an error if the limited buffer is full. He would like to have the option to do different things on overflow.

I'm suggesting adding an enum OnOverflow { DROP_OLDEST, DROP_LATEST, ERROR } to the arguments to allow the user to decide what the overflow behavior should be.

Could onBackpressureDrop() be rewritten to onBackpressureBuffer(0, OnOverflow.DROP_OLDEST) and onBackpressureLatest() would be onBackpressureBuffer(1, OnOverflow.DROP_OLDEST)?

[neerajrj]

+1 Also have an Action0 option to be informed when dropping is occuring.

[srvaroa]

I've got this almost ready, I need some polishing but will be pushing a PR today/tomorrow the latest.