Add ScoobiEnvironment, including support for counters, etc.

Merged manually from:
https://github.com/benwing/scoobi/commit/56964f9c
https://github.com/NICTA/scoobi/pull/123
https://github.com/NICTA/scoobi/pull/145

Conflicts:
	src/main/scala/com/nicta/scoobi/Scoobi.scala
	src/main/scala/com/nicta/scoobi/impl/exec/MapReduceJob.scala
	src/main/scala/com/nicta/scoobi/impl/exec/MscrMapper.scala
	src/main/scala/com/nicta/scoobi/impl/exec/MscrReducer.scala