Move scalastyle-config.xml to dev/ (SBT config still doesn't work) to fix running mvn targets from subdirs; make scalastyle a verify stage target again in Maven; output results in target not project root; update to scalastyle 0.7.0

Author: Sean Owen <sowen@cloudera.com>

Closes #5471 from srowen/SPARK-6861 and squashes the following commits:

acac637 [Sean Owen] Oops, add back execution but leave it at the default verify phase
35a4fd2 [Sean Owen] Revert change to scalastyle-config.xml location, but return scalastyle Maven check to verify phase instead of package to get it farther out of the way, since the Maven invocation is optional
c4fb42c [Sean Owen] Move scalastyle-config.xml to dev/ (SBT config still doesn't work) to fix running mvn targets from subdirs; make scalastyle a verify stage target again in Maven; output results in target not project root; update to scalastyle 0.7.0