Add docs to f7u12.js. Some refactoring too.

Added jsdoc throughout f7u12.js.

Cleaned up a few methods, including breaking up slide() so the
arity is clearly 1 instead of 2, since the second parameter is
only used internally.

The next() method didn't make any sense at all and is much less
complex now and seems to have resolved the merge ordering issues
people have complained about, noted in #2. Will mark it fixed
after it's tested.

Removed the set_name method in favor of directly manipulating
attributes since that's done elsewhere anyways.