BF+RF: refactor base64 and array to text for py3

Refactor base64 imports to use the standard interface.

Replace use of np.savetxt and BytesIO with a small custom function for
writing comma-delimited text.  Fixes Python 3 incompatibility mixing
output from BytesIO with strings.
RF: remove duplicate test

We're testing backwards compatibility in `test_base64_written`
TST: add round trip tests for example images

Add tests for saved / loaded example images.