[NOT READY FOR MERGING] Whiteboard as a "1st class citizen"

This is intended to make the test whiteboard a "1st class citizen", that is, a integral part of the test instance and of the test results.

This work apparently revealed a bug in the combination of avocado's code with Python's `multiprocessing` module.

Sending the object through a queue, and getting it back, changes it in strange ways, such as removing members attributes (whiteboard for instance). This looks to be related to the various guidelines[1] (limitations?) given in the module documentation.

[1] https://docs.python.org/2.7/library/multiprocessing.html#programming-guidelines