Use StaticPool when SQLite databse is :memory:

When SQLite database is memory, any changes will be lost after db is committed unless pool is set to StaticPool.
Reference: http://docs.sqlalchemy.org/en/rel_0_8/dialects/sqlite.html#using-a-memory-database-in-multiple-threads
Merge pull request #227 from chason/master

Use StaticPool when SQLite database is :memory: