Bugfixes and new clock object type

This is a minor release that includes a temporary workaround for a problem
ith dependencies (cherrypy and ws4py) when installing. It also includes
various bug fixes and a new type of Clock object that is useful when
compensating for rendering latencies.

• Bugfix: fixed semantics on calcWhen(), toParenTicks() and toRootTicks()
  when clock speed is 0.` (32256b6)
• API addition: new OffsetClock class to make it easy to compensate for rendering latencies.
• Temporary fix for bug due to changes in cherrypy (f8ec3f6). Will be reverted once resolved by dependencies.