Add _swf.py, which is essentially images2swf.py from visvis.vvmovie

Code is made pep8 compliant, but further barely any changes in
functionality. That will be step 2.
swf seems to work!
finalize swf plugin
tests for added func (iterating over streamed readers)
Retry fetching files on Travis if it fails the first time
address Travis issues
Merge pull request #28 from imageio/swf

Swf