isolate all imports of Subversion modules in svnwrap. editor: move dependancy on Subversion bindings to svnwrap package. In the editor, this involves importing the superclass of `HgEditor' as `svnwrap.Editor'. Additionally, the `delta.svn_txdelta_apply()' function has been abstracted away into a simpler interface, stored in `svnwrap.apply_txdelta()'.