rename holland.core.plugin.manager to holland.core.plugin.loader
rename holland.core.plugin.base to holland.core.plugin.interface
cleanup module docstrings in holland.core.plugin
rename internal plugin loader classes from *Manager to *Loader
implement additional plugin loaders

* Added RegistryPluginLoader that allows registrering plugin classes
  and later load them by (group, name)
* Added ChainedPluginLoader that allows delegating to multiple loaders
  in turn.  This allows loading from a RegistryPluginLoader and then
  trying a more expensive EntrypointPluginLoader.
use ChainedPluginLoader rather than EntrypointPluginLoader

* The default plugin loader now loads from a global plugin
  registry and then tries to find an entrypoint in order
  to more easily support 'builtin' plugins and speed up
  simple builtin-plugin loading
make OrderedDict an alias for SortedDict in datastructures
cleanup loaders
fix various bugs in plugin loaders
disable unreliable shell test