Move source code to the ``lib`` subdirectory. Minor cabal cleanups.
Give up on supporting internal DTD subsets. They're just too complex to
work with easily, and I don't think anybody actually cares about them.
* Remove ``Declaration`` and ``DoctypeNode``.
* Rename ``doctypeExternalID`` to ``doctypeID``
* Add explanatory comment to ``Doctype``, regarding why it doesn't support
internal subsets.
Store attributes in an association list.
Make ``elementText`` and ``nodeText`` return lists, so they work as
standard combinators.
Instance ``Typeable`` for all types
Better support for doctype declarations
Add utility functions for getting node and attribute text.
Reorganized combinator exports
Store element attributes as (Map Name [Content])
Remove mostly useless ``position`` function
Removed mostly useless ``Named`` class
Export ``elementContent`` (herp derp)
Add ``elementContent`` function.
Add ``Ord`` instances for ``Instruction``, ``Name``, and ``Doctype``.