-- Closes ticket #232. --
--
Closes ticket #232.
--

core/packagemanager.py:

	Package.MissingDependency: new exception

	PackageManager.add_dependencies: raise MissingDependency
	exception

	PackageManager.initialize_packages: during dependency
	checking, disable packages that miss dependencies. These might
	still be in _package_list if the depending package failed to
	load for some reason. Also, emit better debug prints.

minesense/VisTrails,VisTrails/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,Nikea/VisTrails,hjanime/VisTrails,celiafish/VisTrails,celiafish/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,minesense/VisTrails,hjanime/VisTrails,hjanime/VisTrails,celiafish/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,minesense/VisTrails,minesense/VisTrails,minesense/VisTrails