Remove IMPORTER from lib/composite.py
Teach build.py to handle relative paths

Previously it was always necessary to be
in the top-level directory for build.py to
work properly. Now, if os.chdir is always
performed to the top-level, and if a build.xml
file is present in the current working dir
(before os.chdir) then it will be passed to
ant as "-f `pwd`/build.xml""

This allows:

  ../../../build.py test

and similar from OmeroPy etc.