Better node removal and root handling

Renames the gem from `llt-tei_handler` to `llt-xml_handler`.

The `PreProcessor` can now be made namespace aware - a root element go to can also be specified. If a namespace is given, the root element also needs to be in that namespace. (a limitiation that might fall in the future).

Works on latin-language-toolkit/llt#18 and latin-language-toolkit/llt-core#12