Making build.xml rewriting way faster

Timing the build.xml rewriting of tomcat-5.5.20-r5

javatoolkit-0.1.9:

real 0m4.944s
user 0m3.869s
sys 0m0.917s

javatoolkit-0.2.0:

real 0m1.485s
user 0m1.262s
sys 0m0.178s

So that gives us a nice 70% speed improvement. It could be even faster if it did not have to do the rewriting twice, but this starts to be fast enough not to bother me too much when writing java ebuilds. The improvement was mainly accomplished by rewriting using expat callbacks instead of manipulating DOM, which in python reads the xml file in using expat any way I think.