Features/177 alembic

This adds a dependency to alembic to handle schema migrations.

By convention, I think it's useful to check in the output of alembic's --sql output so that we can see the DDL that alembic will execute.

I've included those DDL diffs in 39c9b1e

The table creation code has been moved out of ichnaea.db and is only executed for testcases when the database engine is being setup.  All schema changes to the non-testing databases are handled by alembic.