Multidb support

[dcramer]

(This requires #32, and the diff actually will show part of #32 until that is merged in)

Add full multi-db support via --create-all and --execute

This commit does several things:

• Migrations can now be read as path/database/migration.
• Allowed migration databases are now determined by the router, and nashvegas gets created on all capable connections to store state for that connection.
• A new flag on upgradedb, --create-all was added. This attempt will dump creation migrations in path for all databases.
• The --execute flag will now run against all databases.
• The comparedb command now supports the --database argument.

I'd still like to make more improvements, such as bringing in a more South-like interface (e.g. syncdb --migrate, and migrate), as well as incorporate support for the test suite.

[paltman]

Should I merge branches in for both #32 and #33 or are you saying that the branch for #33 includes all of #32?

[dcramer]

33 simply includes #32, so if it were merged #32 would close out as well

You can merge them separate, or just merge #33 :)

[paltman]

Merged, tested locally (I don't have any projects currently with multi-db needs so haven't tested that part of it), and fixed on minor thing to keep things working with migrations at root level. Want to give my changes a quick check to make sure they didn't break what you are doing and i'll publish 0.8 to PyPI.