Patches to also support generating NetBeans's sbt project .classpath_nb file

Hi,

I'm the author of NetBeans support for Scala. During working on sbt integration with NetBeans, I found your great code, and decided to work based on it. Here's the patches that will get NetBeans sbt project descriptor file - ".classpath_nb" to be generated, by adding argument "gen-netbeans=true". 

The major patching work can be summarized as:

1. Scoped classpath entry;
2. A new command argument "gen-netebans", which will create file .classpath_nb instead of .classpath
3. Generate aggregate projects

This work should be compatible with your current work, ie. generate the same result for eclipse.

Cheers,
-Caoyuan Deng