Added the ability to interpolate in the frequency domain to Trace.resample(). Should fix issue #857

Ok, so now here's the promised pull request. The new code uses the numpy.interp() function to interpolate the spectra in the frequency domain. I guess that's roughly what you were suggesting, Lion, right?

A few (in fact two) more thoughts:
1) I'm not sure to what extent the strict_length option makes sense. The endtime of a trace may also change if len(self.data) % factor == 0.0 holds, since the endtime is calculated based on (npts - 1)*delta. 
2) I've seen that in the current master branch there's a new signal.interpolation module, which also contains a weighted_average_slopes() function, of which I'm assuming that it does something similar to SAC's "interpolate" command. Maybe this would in general be the better solution for resample(), since this windowing + filtering business seems a little messy to me. On the other hand the "weighted average slopes" method might be messy too, so not sure...
Thanks for the suggestions and the performance comparison. Indeed, rfft is the better choice of course. I've just merely copied this part from scipy.signal.resample without thinking :-)

I've now changed everything to use fftpack.rfft (commit is yet to follow). Didn't even know that fftpack exists...