add plugins to build.sbt; add support for Output in project configuration; add support for SPARQL Update HTTP parameter

Hi Robert, I made three updates to the code, I hope this helps - it helped in my project :-)

 1. [silk2/build.sbt](https://github.com/ziodave/silk/blob/09038775147fe8fae1ab2aaf2368f5af0e0eaf6c/silk2/build.sbt): added `plugins` to the list of projects in `build.sbt` (I am publishing the artifacts in my local maven repo and without this it doesn't get published),
 2. [silk2/silk-core/.../Dataset.scala](https://github.com/ziodave/silk/blob/09038775147fe8fae1ab2aaf2368f5af0e0eaf6c/silk2/silk-core/src/main/scala/de/fuberlin/wiwiss/silk/dataset/Dataset.scala): restore support for the `Output` tag (it was being interpreted as `Dataset`),
 3. [silk2/silk-core/.../SparqlDataset.scala](https://github.com/ziodave/silk/blob/09038775147fe8fae1ab2aaf2368f5af0e0eaf6c/silk2/silk-core/src/main/scala/de/fuberlin/wiwiss/silk/plugins/dataset/SparqlDataset.scala): add support for `updateParameter` in order to configure the HTTP parameter used to send SPARQL Update queries (defaults to `query`).

Let me know if you need any additional information from my side.

Cheers!