bump version to 13.0dev

Next release will be the thirteenth, so 13 seemed as good a number as any.  It avoids confusion of pyzmq bumping to 3.0 and detaching from libzmq versions at the same time as a libzmq major bump to 3.2.
pyzmq version now only describes pyzmq (semver, etc.)

closes #215
note new version in readme / docs
Merge pull request #303 from minrk/version

bump version to 13.0dev

closes #215