Return exception string in place of throwing an exception

Big bunch of `AttributeError`s when renderer failed to be created that renders
vim unusable in case of some of the errors (e.g. invalid configuration) is
annoying.

Conflicts:
	powerline/__init__.py
Use `try/except KeyError` in place of `if in dict/else`
Pop path from self.loaded on exception
Replace self.pl.exception with self.exception

New function is checking for self.pl being set before using logger and also
removes the need of specifying explicit prefix="powerline".

Conflicts:
	powerline/__init__.py
Remove cache before running self.exception, not after
Fix Powerline.exception
Use fallback logger if normal one is not available
Do not double pl.exception() call, record used pl in variable instead
New fixed segment showing position of current view

Solves #539
Fixed version of #540 and #579
Works now for split windows and wrapped lines.
Add powerline/bindings/fish/powerline.fish
Make fish bindings feature complete, add documentation

Added features:
- Updating TMUX_PWD_* when PWD variable changes
- Updating TMUX_COLUMNS_* when WINCH signal is received
Add support for number of jobs to fish bindings
Merge branch 'develop' into feature/539-position-segment
Small fixes for new position segment

- use vim_funcs
- use a dictionary instead of a tuple for translation strings
Add fish tests
Improve test_shells/test.sh:

- Add argument that makes it run only one shell
- Make this argument also force skipping removal of tests/shell directory
- Make it copy logs to tests/shell before and after processing
Make postproc.py only leave fish prompt

Should fix fish tests
Another attempt to fix fish on travis
More test cases for position segment
Merge pull request #750 from EinfachToll/feature/539-position-segment

New (fixed) segment showing position of current view