A few bug fixes and a configuration option for page and post priority

Before I could add my feature, I had to fix a few bugs so it would pass all the rspec cases. I'd rather not do a pull request for everything in one lump, but I'm not exactly sure how to do it. So here are the changes I did:

1. Update the specs to work on rspec 3.0: be_true is now be_truthy
2. Correctly format timestamps of static files: Use %S for seconds of the minute instead of %s for seconds since the UNIX epoch
3. Allow overriding priority for pages and posts
4. Document the above option I implemented and the existing option to exclude a page or post from the sitemap