Improve MovableType importer

I made a series of tweaks to the Jekyll importer for MovableType:

- Use Sequel dataset instead of a hand-crafted SQL query. This makes it easier to access all the DB columns without specifying them directly.
- Don't publish unpublished blog entries
- Export file names with YYYY-MM-DD instead of non-padded dates.
- Don't force hyphens in file names, which can break permalinks that used underscores.
- Export excerpts for blog entries.

I'm happy to make any changes you'd like to see for this pull request. I have some tests for the importer stashed presently; I couldn't get `rake test` to run on master, so I thought I'd hold off for now.