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)
Add Tobias Bengfort to AUTHORS
pep8
Fix rst in CHANGES
add missing files to doc TOC
document release automation
document semantiv versioning exception
Add 2.0.0 date to changelog
Merge pull request #696 from nidico/feature-219-alembic+update-sqlalchemy-0.9

Use alembic instead of sqlalchemy-migrate; update to sqlalchemy 0.9
Add CHANGES for 2.1.0
Remove explicit authorship without significant contributions

Note that there are more unnamed authors with more contributions.
pep8
Fix errors in INSTALLATION.rst