Enable & configure gradle eclipse-wtp plugin.

This PR suggests applying the `eclipse-wtp` plugin for IMHO easier Eclipse integration.

In a fresh clone of mapfish-print, one can then do the following:

* (on the command line) `./gradlew eclipseWtp` (possibly also: `cleanEclipse` and `cleanEclipse`)
* (in Eclipse) "Import existing project" ⇒ find your folder
* (in Eclipse) right click on the project ⇒ "Run as..." ⇒ "Run on server"
* (Select or download a server)
* visit `http://localhost:8080/print-servlet/` or `http://localhost:8080/print-servlet/pdf/info.json`

This is a common workflow for me. It may also work for others.

I am not very familiar with gradle; I hope I did everything "the right way"™. 

Please review.