Maven redux

Here's my initial work to get Nengo building in Maven.  It seems like there's still some small issues with the Nengo-UI project, but other than that it does build in netbeans (and should build in eclipse, too).

The dependent projects abeille, jmatio, and ssj have all been put in the sonatype public repository, so we should be able to eliminate just about all of the explicit jar dependencies.  I've also rearranged the source files into the standard maven format; I hope this wasn't too much shuffling too early.