Deal better with database connection loss (Issue #6)

[oz-linden]

* Wrap all database operations (including reads) in an SQLAlchemy transaction
  per http://docs.sqlalchemy.org/en/latest/orm/session_basics.html#when-do-i-construct-a-session-when-do-i-commit-it-and-when-do-i-close-it
* Add configuration parameters to set the pool_recycle and pool_pre_ping engine parameters
  defaults are to recycle in 30 minutes and to always ping

I've been running with this in my errbot using an AWS RDS db storage backend for a little while now and have not seen the errors I was getting before when RDS disconnected.