Add memcache increment and decrement

This is a fix for #1083. It looks like the suggested changes for memcached are already done in 2.0 (if you want to I can prepare a PR against that too). I'm sorry about the formatting issue for the PR review, tried to fix it but seem to be screwed up anyways ... I was not sure where to send the PR against and how to handle phalcon.c in a PR. Since incr and decr are atomic, this is very useful i guess.

I thought about error handling in that implementation, but increase or decrease a non-numeric value simply raises a notice (which I was not able to prevent without shutup in userspace) and a not-existent value returns false. I can't really think of any other error-related use case.