Update maven and eclipse support

1) Maven and eclipse support have been improved over the years.
=> Eclipse integration is transparent when using the scala IDE (which is based on eclipse).
=> As for maven, the reference plugin has changed (maven-scala-plugin is now scala-maven-plugin).

see http://scala-ide.org/docs/tutorials/m2eclipse/

2) Improve stability of the build.
=> set up encoding explicitly
=> set up prerequisite maven explicitly
=> set up version even for plugins
Merge pull request #8 from BertrandDechoux/pom-update

Update maven and eclipse support