Add removeCallback operations to the MetaDriver.
Change executeAsync signature to return the query identifier.
Add a new parameter --async to MetaSH to activate asynchronous query execution.
Modify readme to include async execution
Merge branch 'streaming-integration' of github.com:Stratio/stratio-meta into streaming-integration

Conflicts:
	meta-core/src/main/java/com/stratio/meta/core/engine/Engine.java
	meta-sh/src/main/java/com/stratio/meta/sh/Metash.java