Prepared plugin codebase for merging

I started preparing the repository-connector-plugin to merge the repoclient-plugin:
https://wiki.jenkins-ci.org/display/JENKINS/Maven+Repository+Client

We discussed that a while ago and I finally got the time to put some work into it:
https://groups.google.com/d/topic/jenkinsci-dev/ssk0vM3zUqM/discussion

Here is what I did so far:
* Sort the list of Maven repositories in the global configuration page
* Added I18N and translated all help files and texts to german
* Extracted the global configuration from the ArtifactResolver and moved it into the central class GlobalConfiguration
* Added the version selection build parameter
* When your ArtifactResolver build step and the build parameter is used in one job, the version, selected by the builld parameter overrides the version of the ArtifactResolver build step.
* Added proxy support which makes the plugin usable in corporate environments