[SPARK-6782] add sbt-revolver plugin

to make it easier to start & stop http servers in sbt
https://issues.apache.org/jira/browse/SPARK-6782

Author: Imran Rashid <irashid@cloudera.com>

Closes #5426 from squito/SPARK-6782 and squashes the following commits:

dc4fb19 [Imran Rashid] add sbt-revolved plugin, to make it easier to start & stop http servers in sbt