Add renderer docstrings.
Remove `pl` argument and first argument to `render*` from docs
Fix documentation for ThreadedSegment classes with @staticmethod’s
Rename PowerlineState to PowerlineLogger
Replace various __import__ calls with one Powerline.get_module_attr

Also makes some more errors non-fatal:
- Failure to import renderer class in case there is an existing renderer object
- Failure to import segment function
- Failure to import matcher function

One of the purposes: create a function that is able to collect all imported 
modules to reload them (really purge out from `sys.modules` and let the python 
do its job when reimporting powerline and recreating Powerline and other 
objects).
Remove reference cycle

Switches from

    powerline ↔ get_module_attr (as a bound method it contains reference to
            │       ↑            self)
            └────> theme

to

            ┌────> pl, import_paths
            │       ↑
    powerline → get_module_attr
            │       ↑
            └────> theme

This is a separate commit because I am not sure whether it makes sense to bother 
with this cycle.
Add set_event argument to .shutdown() method