Moving Mock classes to a more convenient/accessible location: inside lens-server-api
Query Priority implemented.
    * Change in DriverSelector's api to facilitate explain on all drivers is called beforehand.
    * Change in QueryContext,PreparedQueryContext and addition of DriverSelectorQueryContext, AbstractQueryContext and ExplainQueryContext in order with the last point
    * Priority Decider API and implementation for Hive Driver
    * Hive Driver modifications to use Priority Decider API. Only called in async execute and surrounded with a try block.
    * Accompanying test cases
renaming some config variables and adding default values in conf xml
Added javadoc and apt files.
Merge pull request #162 from prongs/job_priority

set mapred job priority by hive driver