HOTFIX: Clean up build in network module.

This is currently breaking the package build for some people (including me).

This patch does some general clean-up which also fixes the current issue.
- Uses consistent artifact naming
- Adds sbt support for this module
- Changes tests to use scalatest (fixes the original issue[1])

One thing to note, it turns out that scalatest when invoked in the
Maven build doesn't succesfully detect JUnit Java tests. This is
a long standing issue, I noticed it applies to all of our current
test suites as well. I've created SPARK-4159 to fix this.

[1] The original issue is that we need to allocate extra memory
for the tests, happens by default in our scalatest configuration.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #3025 from pwendell/hotfix and squashes the following commits:

faa9053 [Patrick Wendell] HOTFIX: Clean up build in network module.