Major update of core module.

- Group functions logically, and rename several functions to be more
  consistent.

- Drastically improve namespace handling, with several new public
  functions and updating access and update helper functions to be
  namespace-aware.

- Replace the disjointed set of access and modification helpers with a
  consistently-named and ~orthogonal set, rewritten with much cleaner
  implementations as well as namespace awareness.

- More comprehensive tests.

The new modification functions:

- alter-tag, alter-attrs, alter-children -- Modify an SXML element by
  applying a function to the tag, attributes, or children.

- replace-tag, replace-attrs, replace-children -- Modify an SXML element
  by replacing the tag, attributes, or children with a constant value.

- update-attrs -- Modify an SXML element's attributes by merging new
  attribute values into/over the existing ones.

- map-children -- Modify an SXML element's children by mapping a
  function over the child elements.
Delete the now-unused util module.

It contained one function `any-of`, which is now obsoleted since Clojure
1.3 by `some-fn`. All users of `any-of` have already been migrated in
the previous commit.
Use `juxt` more, just because.
Update README for recent changes.

Add documentation for new SXML manipulation helpers, strike off the
"namespaces are annoying" TODO, and update some example outputs to
be consistent with recent changes to transform module.
Release 0.2.0, why not.