removed the fft_type option from convolve as per issue #660.  User can
specify their own fft; there's no need for astropy to test it.
refactor != -> np.isnan, len(shape) -> ndim
cleaned up tests
removed residual scipy junk (had considered testing scipy... no need)
convolve_dtype option added (no reason to force users into 128)
added warning about ignoring edge zeros etc. when kernel not normalized
fixed lost lines.  I don't know what happened still, but I think it has
something to do with checking out branches, loading branches into the
local vim buffer, then switching branches again without reloading
tiny fixes
restore numpy's default complex dtype
restored a lost comment
warnings -> log
added discussion of when to use your own fft and an example
only warn if options set
Merge pull request #660 from keflavich/convolve-npfft

Should we use PyFFTW instead of PyFFTW3 for astropy.nddata.convolution?