Use different emulate call

Ref #389
Also take tests from @kovidgoyal develop branch
Add a note about vim-addon-manager
Forbid querying vim from non-main threads in tests

Currently tests nothing or almost nothing.
Move some thread functions to a separate class
Remove debugging function
Make it use existing shutdown events

Also adds check to powerline-lint that "args" dictionary does not set
shutdown_event, segment_info or pl keyword arguments, this will lead to problems
Replace MultiClientWatcher and Powerline threads with ConfigLoader

Also
- move file opening and parsing to ConfigLoader
- add interval configuration
Defer create_renderer (and thus thread creation) until .render call

May fix problem discussed in #397

Also fix documentation for Powerline.render().