this merge contains:
* a new plugin architecture
* a pluginized pytest core
* many pytest related refactorings
* refactorings/streamlining of pytest's own tests
--HG--
branch : trunk
provided in the exportdefs, copied to the namespaces (from whereever they
come), and added docstrings to all exposed namespaces (except for _thread for
now).
--HG--
branch : trunk
hpk