Added methods to convert S4 object to an ape tree and back

includes inst/tests/test_ape.R to illustrate and test these
methods.

While higher-level than the XML-to-S4 methods so far, these
methods are still not part of the top-level API, which will
work at the document level with user-friendly functions.
Added test file illustrating how the top level API might work

This test won't yet pass, since these functions have not yet been
written.  Illustrates what the API might look like and shows a
test for validating against the schema.