DFTFilter Class implemented

#353

So I have made a DFTFilter class, with methods to create different types of filter given width and height of the filter. These filters can be reused with other images of similar dimensions by img.applyDFTFilter(filter) . Also, I have included a test file in the examples/manipulation folder.
P.S. Sorry, I could not clean up my previous commits. Please ignore other commits except this latest one.