Support Python 3 asyncio concurrency

[manics]

Adds support for creating an OMERO session and Ice services that can be used with asyncio.

• 00707b7 creates an exact copy of createSession called createAsyncSession
• a6583d0 modifies createAsyncSession to create an async session

Example:
https://gist.github.com/manics/22583d9404022f1b57df0e40f97b4cd3

[manics]

Does anyone have opinions on how to support this usage? I initially implemented it as a separate python module, but the async version of createSession, createAsyncSession, is problematic to implement outside this module. createSession uses __private variables, so re-implementing this method in another class requires one of:

• break the encapsulation and unmangle the private variables: e.g. self.__sf is referenced as self._BaseClient__sf, see for example https://github.com/manics/omero-asyncio/blob/4515a76a2ce9e7631b4bdb9ba3d2fb8829b97a65/omero_asyncio/omero_asyncio.py#L135-L276
• duplicate all private variables and methods called by createSession: createAsyncSession has to call other class methods that also reference some __private variables, so if those private variables are duplicated in the subclass then all methods that reference them must also be duplicated
• refactor createSession to support overriding: potentially dangerous given the complexity of the existing code

[joshmoore]

I'm for (3) refactoring. Happy to help.