Use prxml for more delicate writing of pom.xml

[amalloy]

ant isn't very flexible about how it writes the pom.xml; in particular it doesn't do a very good job with :repository elements. At ninjudd's request I used c.c.prxml to create the pom.xml file so we don't have to depend on that ant task anymore: it's implemented as a multimethod that handles most elements just fine, and has hooks for customizing the handling of others.

[amalloy]

Note: I don't think it handles dependencies with exclusions yet (it ignores the exclusions), and might not handle license/dev-dependencies either.

[lancepantz]

thanks alan! I'm going to refactor a little bit after some updates the defile task