- reworked the plugin loading mechanism

Using the "old" exec approach isn't possible anymore because it'll break all
plugins which aren't compatible with python3 (for instance the usage of
python2's "print" statement will lead to an error). In order to circumvent
this problem we do the following:

- import the plugin/module
- update the module's global symbol table with the "globals()" of the
  commandline module
- bind the module's "do_*" functions to the "Osc" class

This basically mimics the old "exec" semantics.
- removed plugin migration hint from the NEWS file (for now)