Use alembic instead of sqlalchemy-migrate (fixes #219)

As sqlalchemy-migrate isn't actively developed anymore, it is
recommended to switch to alembic. Our previously used version of
sqlalchemy-migrate wasn't compatible with SQLAlchemy 0.8 and thus
prevents us from updating, and the latest version doesn't build with
buildout (see [0]). Therefore let's update now.

This commit
- installs alembic instead of sqlalchemy-migrate
- initializes an alembic environment below src/adhocracy/alembic
- adjusts the code which is called from `paster setup-app`
- leaves the old sqlalchemy-migrate migrations in place (for now; we
could probably also just remove them)

In order to migrate to alembic, we need to make sure users update to a
git tag (e.g. version 1.9) before this commit, perform their remaining
sqlalchemy-migrate migrations, and then move to this commit or newer and
start using alembic.

[0] https://code.google.com/p/sqlalchemy-
migrate/issues/detail?id=165&sort=-id
Add MutableList (required for SQLAlchemy >= 0.8)
Update SQLAlchemy (0.7.10 -> 0.9.1)
Merge pull request #696 from nidico/feature-219-alembic+update-sqlalchemy-0.9

Use alembic instead of sqlalchemy-migrate; update to sqlalchemy 0.9