Replaced subclassing of OpXToMulti with a factory.

Based on pull request by Kemal Eren.

https://github.com/ilastik/lazyflow/pull/73
remove unused class
remove dead code
refactor: break up operators.py into individual files
OpArrayCache: use new-style slot declaration
cleanup: OpArrayCache
document the purpose of OpArrayCache
minor test fix
remove OpTransposeSlots class
cleanup of generic.py