Remove unused imports
Use a global SessionMaker and per-request sessions

This fixes the following error caused by a race inside SQLAlchemy when using different session
makers:

    AttributeError: 'NoneType' object has no attribute 'twophase'

We also merge phantomsql code inside Phantom, since the code is very
small and the added abstraction layer makes it more complicated to
understand.
Merge branch 'master' into phantom0.2