comparedb improvements (ignoring, more context, error handling)

I've noticed a couple of issues with nashvegas (comparedb):

- it doesn't always provide enough content to find which table has changed (and it's not always trivial to figure out what the table name should be) -> added a "-l" option to provide lines of context
- it provides noise by comparing irrelevant data such as comments and "ADD CONSTRAINT" lines. These can be selectively ignored using a "-i <level>" flag
- if something is wrong with a migration (e.g. an empty .sql file that I was about to write), comparedb no longer works. But more annoying, the compare database is not removed; fixed this.

Additionally, syncdb is make do work again somewhat using the --skip-migrations flag. This allows comparedb to work (create new database+syncdb) if there are migrations (which cannot be applied before the real syncdb has taken place)