Creating pre task to save new scala-ide-play2 prefs

Hi, there is a pull request https://github.com/scala-ide/scala-ide-play2/pull/43 that will improve the experience of working with Play and Eclipse. This is about adding imports to the html templates compilation. The problem is that in the current form you need to keep in sync both Build.scala and your eclipse configuration manually.

With this change you will be able to create the file org.scala-ide.play2.prefs during the execution of "eclipse" command. I used the pre tasks execution mechanism of sbteclipse.

Mention to @skyluc to get some help in the review.