Add support for TEI P5

[jhellingman]

The tei2html stylesheets have been developed with the now very old TEI P4 version of TEI. The stylesheets should also work with TEI P5 and later documents. For this a few changes need to made:

1. Handle TEI top-level element (DONE)
2. Handle new style of REF elements, for both internal and external links (DONE)
3. Handle CHOICE elements for corrections, etc. (also list of errata needs to be adjusted). (DONE)
4. Handle removal of DIV0 element (will work by default, but may need some tweaking).
5. Correctly handle metadata (when dealing with TEI namespace). (DONE)
6. Correctly handle TEI documents that use the TEI namespace. (DONE)
7. Correctly handle external references in table of external references.
8. Verify usage of xml:id and xml:lang attributes is correct.