Added schema_* operations to backends, kvdb_paired supports mixed persistency
kvdb_ets:schema_write() to return 'ok'
kvdb_ets:schema_delete() to return ok
Merge remote-tracking branch 'origin/uw-1.2.1'

Conflicts:
	rebar.config
	src/kvdb_ets.erl
	src/kvdb_leveldb.erl
	src/kvdb_paired.erl
error in decode_write