convolve/correlate first versions done

Hi David,

I've finished my first pass at convolve/correlate. I'm interested to hear what you think of the syntax, especially for option passing with case classes. The options are all placed in breeze.signal/options.scala

These are some examples of how one would use convolve/correlate, as I have it now...

    val kernel = DenseVector(1.0, 2.0)
    val data = DenseVector(2.0, 3.0, 4.0, 5.0)

    //default has no overhangs
    convolve(data, kernel)  =>  DenseVector(7.0, 10.0, 13.0)

    //this is the MatLab default, with zero padding on both ends (i.e. padding = OptPadding.Value(0d) )
    convolve(data, kernel, overhang = OptOverhang.Full() ) => DenseVector(2.0, 7.0, 10.0, 13.0, 10)

   //one can also specify padding with the boundary values (first and last element of the data)
   //which is sometimes done in DSP, especially image processing....   padding = OptPadding.Boundary()

   //one can also specify cyclical padding, which is also common in DSP...
   //padding = OptPadding.Cyclical()


I'll edit if you have any comments, and then I plan to document it all with scaladoc and in the Wiki.
At some point, I'd like to put in fft convolution for longish floating-point DV's
     convolve(  data,  kernel,  ...,  method = OptMethod.FFT()  )

Kenta