Updated contributor agreement

This is a follow up to our contributor agreement being out of date (the Open Planning Project changed its name to OpenPlans a while back). This was noticed back during OSGeo graduation, and was discussed in our GeoServer meeting earlier in the year.

I had an opportunity to ask for a replacement earlier this week. The agreement was provided as "GeoServer assignment agreement as of June 2012.doc". I have saved it out as a PDF for our developers guide.

Update: Fixed a few warnings in the developers guide.