rez.plugin_managers: simplify code and quarantine yapsy in preparation for extermination.
rez.plugin_managers: create a master plugin manager class which is the only public interface for working with registered plugins.  this is more consistent and predictable than having a separate manager per plugin type.
replace yapsy with a custom plugin loader.

Custom plugins are organized under a python package named 'rezplugins'.
The direct sub-packages of 'rezplugins' are the known plugin types supported by
rez, and the modules under that are indivudal custom plugins extending that type.
rezplugins: No need to expand the search __path__ directly under rezplugins, because it contains the complete, static list of plugin types:  these cannot be added to by end users.
rezplugins: improve documentation
rezplugins: bugfix to setup.py
Merge branch 'origin/plugins'

Conflicts:
	setup.py
	src/rez/plugin_factory.py
	src/rez/plugin_managers.py
	src/rez/rezconfig
	src/rezplugins/build_system/cmake.py
	src/rezplugins/release_vcs/git.py