Some updates to keep the yedit feature up to date

This pull request updates the following dependencies:

* The p2 repository was changed form indigo to kepler
* Tycho version was bumped from 0.15.0 to 0.18.1
* Snakeyaml version was bumped from 0.8.0 to 0.13.0

In addition to that, the build was changed to:

* create an p2 update site. The project was already present but it didn't seem up to date (no pom file and a legacy style site.xml)
* obtain the snakeyaml jar using a standard maven dependency instead of having it checked in together with the source. The jar that is brought in when resolving this dependency is an OSGi bundle and was added to the yedit feature. That in turn means that it is included in the generated p2 update site.