Generate source jar by default and use specific the maven-jar-plugin version.

It's always nice to have the source to a project and although they easily included with ```mvn source:jar install``` I'd prefer it included by default.

Got the following warning from maven when installing:
[WARNING]
[WARNING] Some problems were encountered while building the effective model for se.lth.cs.nlp:wikiforia:jar:1.0-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-jar-plugin is missing. @ line 38, column 15
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING]

So I cleaned it up.
![selfie-0](http://i.imgur.com/3GGeFUz.gif)