Add Scalastyle linter that ignores output.

Parse Scalastyle lint output.

Add a Scalastyle linter

Summary:
I've been using Phabricator a lot recently for Scala projects. We thought it might be useful to add linting to our code review process, and I didn't see any current Scala linters in Arcanist, so I put together a Scalastyle linter. I didn't find existing revisions or tasks related to Scala linter configuration, but if there are I'm happy to wait or contribute to that instead of pushing this change.

I'm not too familiar with Arcanist contribution processes; please let me know if I'm doing anything very wrongly!

Test Plan:
I ran this linter locally.

I tried adding a unit test but I wasn't sure how to configure the paths to the Scalastyle JAR and configuration file in the test environment. Are there examples of other tests that do this?

Reviewers: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10405

Add check for empty stdout.
Merge pull request #1 from josh-newman/josh-scalastyle

Add Scalastyle linter