Kalman filter

Hi there--I've written a basic Kalman filter to add to the package.  It's still a little rough and I haven't tested it very rigorously, but it's working.  This could be useful for building time series modeling functionality: R uses MLE based on a Kalman filter as the engine behind their standard "arima" function.

Take a look and let me know what you think; there's a test script in there too that shows the basic usage.

Cheers,

Sam