avocado.core: Introduce tree library

Introduce a new data structure that will be the base of
our new multiplexing mechanism.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado: Introduce the new multiplexer library

Based on the new treenode data structure, we
built a new multiplexing algorithm. It has some
different properties and uses a more standard
YAML file format for the configuration files.

This commit is the new base library.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado: Change framework to use the new multiplexer

That change has to be made all at once for the sake of
bisectability. Update avocado.job and the multiplexer
plugin with the new multiplexing library code.

Also, update functional tests, the avocado sample tests
with the new YAML based config files, and the requirements.txt
file with the new prominent dependency of avocado, PyYAML.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado: Remove the old multiplexer

Signed-off-by: Ruda Moura <rmoura@redhat.com>
docs: Update documentation with new multiplexer sections

Also, remove the references to the old system.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #159 from lmr/new-multiplexer-rmoura

New multiplexer