Docx Reader: A nicer Docx type.

This modifies the Docx type in the parser to avoid all the extra files
(Notes, numbering, etc). A reader monad keeps track of these, and applies
them at the end. The reader monad is stacked with ErrorT to enable better
error-handling than the old Maybes. (Note that the better error handling
isn't really there yet, but it is now possible.)

One long-term goal of these changes is to make it easier to write the Docx
type. This should make it easier to develop a standalone docx package in the
future.
Docx Reader: Add a compatibility layer for Except.

mtl switched from ErrorT to ExceptT, but we're not sure which mtl we'll
be dealing with. This should make errors work with both.

The main difference (beside the name of the module and the monad
transformer) is that Except doesn't require an instance of an Error
Typeclass. So we define that for compatability. When we switch to a
later mtl, using Control.Monad.Exception, we can just erase the instance
declaration, and all should work fine.