#342 Run nosetests in parallel

This branch adds the ability to run nosetests in parallel with the same number of jobs as scons uses for compiling.

It has been simmering for a while, since I was waiting for some other branches to be merged, but Barney had been having trouble with it on a Mac 10.7 machine.  I haven't been able to find any systems that fail (including Macs with 10.6 and 10.8 -- I don't have a 10.7 machine), so I figuring I'd put it out there and see who else might have trouble with running the tests in parallel in the hopes that it might help diagnose the problem.

Basically, you should just run 'scons tests' and see if it completes.  If you have multiple processors, it should run through faster than normal.

If it fails, please do:
```
scons install
cd tests
nosetests --processes=4 --process-timeout=30 -v
```
(Or some other number than 4 if you have a different number of processors.)

The -v will output which tests it actually tries to run, so perhaps we can figure out which test has trouble being run in parallel.