Add 'scala-jar' to the list of jar artifacts.

The CustomPomParser has a hard-coded list of 'blessed' Maven POM packaging types
that are actually packaged as jar files, but Ivy incorrectly maps the file extension.
This patch allows artifacts published with the 'scala-jar' packaging to be properly
resolved and used in an SBT project.
Add a unit test for the 'scala-jar' mapping.
Make jar artifact a 0-byte file.
Merge remote-tracking branch 'upstream/0.13' into 0.13
Merge pull request #1400 from dpratt/0.13

Add 'scala-jar' to the list of jar artifacts.