For #36: Publish snapshots to jcenter.

 - Still needs encrypted `BINTRAY_USER` and `BINTRAY_KEY` added to the
   `.travis.yml` file.
Add artifactory plugin.

 - Consolidate plugin declarations into the new `plugins {}` closure.
 - Also add the `java-gradle-plugin` which is supposedly handy for
   gradle plugins.
Merge branch 'publish_snapshots' of github.com:dougborg/gradle-node-plugin into dougborg-publish_snapshots

Conflicts:
	.travis.yml
Merge branch 'publish_snapshots' of github.com:dougborg/gradle-node-plugin into dougborg-publish_snapshots

Conflicts:
	.travis.yml
Merge branch 'dougborg-publish_snapshots'
Added artifactory snapshot repo url.