Build with Maven and Tycho

Hi! First of all, thanks for the nice Turtle editor! We'd like to use it in the [Culturegraph Metafacture IDE](https://github.com/culturegraph/metafacture-ide), but we can't consume your published update site from our build.

This commit adds a setup to build the Xturtle editor with Maven and the Tycho plugin (call `mvn clean install` in the repo root). The build creates a p2 Eclipse repository in the `updatesite` project (under the `target` directory). This p2 repository makes the editor consumable by other Eclipse features, e.g. by pointing to it from a target platform definition in another Tycho build (which is what we do).