Refactoring to ovation-api + ovation packages

It'd be great to separate the ovation module (pure Python) from the
ovation-api (us.physion.ovation…)
Separate ovation_api module and ovation modules

Update ovation.__init__ to import necessary modules, and make
associated import changes in pure python modules.
API package corrected
Merge pull request #2 from physion/package-refactor

Package refactor