[GEARPUMP-246] clean up 'examples/target' when invoking 'sbt clean'
[GEARPUMP-246] clean up 'examples/target' when invoking 'sbt clean'

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

 - [x] Make sure the commit message is formatted like:
   `[GEARPUMP-<Jira issue #>] Meaningful description of pull request`
 - [x] Make sure tests pass via `sbt clean test`.
 - [x] Make sure old documentation affected by the pull request has been updated and new documentation added for new functionality.

Author: huafengw <fvunicorn@gmail.com>

Closes #120 from huafengw/fixClean.

manuzhang/incubator-gearpump,manuzhang/incubator-gearpump,manuzhang/incubator-gearpump,manuzhang/incubator-gearpump,manuzhang/incubator-gearpump