Introduce Alembic for H

It'll be used for migrating our user db, upgrading structure changes

This commit contains the basic architecture and the updated requirements.
Alembic new migration file: user subscriptions

The upgrade part created the user_subscription table and alters the user table to add the subscriptions column.
The downgrade part drops this new column and this new table
Alembic uses postgresql connect string

We're using a postgresql user db in the production and also currently alembic has issues with adding a new column for an existing table for sqllite
Add alembic settings to development.ini and setup.py

By default alembic creates its own ini file, but with the -c flag any ini file can be used.

New section added: alembic
New log section added: alembic
Move alembic scripts to h.migrations
Merge pull request #1081 from hypothesis/1078-set-up-alembic-3

1078 set up alembic