ENH: Speed up fftconvolve by using 5-smooth length to pad FFT

Fix for https://github.com/scipy/scipy/issues/2146

```
In [4]: in1 = rand(2**14+1) + 1j*rand(2**14+1)
 
In [5]: timeit fftconvolve(in1, in1)
100 loops, best of 3: 19.1 ms per loop
 
In [6]: timeit fftconvolvefast(in1, in1)
100 loops, best of 3: 3.75 ms per loop

In [10]: assert_array_almost_equal(fftconvolve(in1, in1), fftconvolvefast(in1, in1))
 
In [11]: 19.1/3.75
5.093333333333334
```