DFT class created

Issue #353

```python
flt = DFT.createGaussianFilter()
flt = DFT.createButterworthFilter()
flt = DFT.createLowpassFilter(xCutoff=40)
flt = DFT.createHighpassFilter(xCutoff=40)
flt = DFT.createBandpassFilter(xCutoffHigh=400, xCutoffLow=40, size=(512, 512))
flt = DFT.createNotchFilter(dia=50, size=(400, 300))

flt1 + flt2 # adds two filters
flt.invert()
flt = flt1.stackFilters(flt2, flt3) # Stacks 3 single channel filters to create one 3 channel filter
img = Image('lenna')
flt.applyFilter(img).show()
```

 - 3 Channel support for all the filters
 - Filters are created using numpy arrays
 - SimpleCV image of the filter can be obtained