Add option to automatically encode/decode strings with byte format

[nickgaya]

Define a SwaggerFormat for the "byte" string format that base64-encodes binary data to wire and base64-decodes from wire to a Python bytes object.

This behavior is not backward-compatible so I added a config option to enable it, disabled by default.

Related: #146

[coveralls]

Coverage increased (+0.002%) to 98.893% when pulling 2a0e6c0 on nickgaya:byte-format-base64 into 16e7529 on Yelp:master.

[nickgaya]

@macisamuele, @lucafavatella: Any thoughts on this?

[sjaensch]

This is great, thanks! We should probably default the config value to True in the next major release, and then start a deprecation process. Not using base64 goes against the spec.