allow different read/write resources in the same job
Split global resource into read/write targets

Improve conf to allow for dedicated read and write resource as oppose to
a single, unified resource used for both. This allows for different ES
indices to be used in the same index, one as a source and the other as
a sink.

'es.resource' is still supported and used as a fall back.
Higher level abstractions, such as Cascading, Hive and Pig, set the
proper property automatically.

fix #156
fix #45
fix #26