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 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
fix html escaping in search placeholder text

cgi will not encode '"'.
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
Merge pull request #740 from xi/feature-fix-search-escape

fix html escaping in search placeholder text
pep8
Minor dependencies upgrade
Build .mo files automatically from buildout

Note: This uses c2c.recipe.msgfmt which seems to be unmaintained.
rm .mo files (fixes #738)