Renamed pypjua._pjsip module to pypjua.core and split it up into several files

darcs-hash:20090107174009-58103-d3ec8360d4a5b4290fd8d09d6a197264215b667a
Updated install docs to include new new .pyx file name

darcs-hash:20090108100633-58103-c8eed176d3cc5d278d948e72264e86f251819199