Production Release of imageZMQ adding new methods

New methods were added:

• Added close method in ImageSender and ImageHub classes.
• Added __enter__ and __exit__ methods so that ImageHub and
  ImageSender will work when used in a with statement.
• Minor bug fixes and documentation updates

First production release of imagezmq

This is the first production release of imagezmq after 2 years of testing. This release is stable. Backwards compatibility will be maintained. If you discover any problems, please open an issue at https://github.com/jeffbass/imagezmq/issues.