Don't show '//' when using path separator

Currently, the cwd segment is showing //etc when we are in /etc and / when we are in the root file.
This small patch fix this
Fix display for home directory
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
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
Also output full.log

Hope it will give some hints about why fish tests fail on travis
Make XDG_CONFIG_HOME point to writeable location
Use `.` for sourcing in fish

It appears that travis uses fish version without this command. Though `.` is 
deprecated on new versions it is still supported for backwards compatibility, 
thus may be used here.
Also print shell version string
Disable fish tests on travis
Merge pull request #696 from ZyX-I/fish

Add powerline/bindings/fish/powerline.fish
Add tests
Merge branch 'patch-1' into develop

Fixes #724