More/improved docstrings for spreads.config
Docstrings for spreads.metadata
Rename HookPlugin subclasses for more coherence

The now all comply with the *HooksMixin naming pattern, which avoids
confusion (previously there were *HookMixin and *HookMixins).
Remove `path` parameter from `prepare_capture` signature
Small PEP8 fixes
Rename DevicePlugin to DeviceDriver
Docstrings for spreads.plugin