Update font patching documentation

Update reference to where fontpatcher.py is located
Use `stty size` or zero as a fallback for $COLUMNS

Fixes #924
Merge pull request #926 from ZyX-I/fix-cygwin-COLUMNS

Use `stty size` or zero as a fallback for $COLUMNS
Do not define POWERLINE_COMMAND when not using prompt

To be consistent with fish and bash
Merge pull request #927 from ZyX-I/move-POWERLINE_COMMAND

Do not define POWERLINE_COMMAND when not using prompt
Rename some bash powerline functions

Renamed `_powerline_prompt` to `_powerline_set_prompt` and `_run_powerline` to 
`_powerline_prompt.`

Reason: name `_run_powerline` is not consistent with other names used in shell 
bindings.
Refactor _powerline_init_tmux_support

- Move _powerline_tmux* functions definitions out of
  _powerline_init_tmux_support.
- Do not touch PROMPT_COMMAND unless tmux support is to be added.
Add various other shell implementations support

Tested in busybox, mksh, ksh and dash. Ksh has issues described in 
troubleshooting.
Fix shell tests: VIRTUAL_ENV is not accessible by powerline by default
Disable libzpython support in zsh
Add tests for new functionality
Fix travis build: on travis “Terminated…” message is truncated
Document why `stuff` and waiting are used for dash
Merge branch 'sh-support' into develop

Closes #928
Add missing imports in powerline-config

They matter when script is used without properly installing powerline, so that 
importing `powerline.bindings.config` does not work.
Merge branch 'fix-powerline-config' into develop

Ref #930
Make sure arguments in `args` are only strings

Required for python-2.6

Fixes #933
Merge branch 'fix-933' into develop
Also check error messages
Ignore IOError’s in ConfigLoader.update

Fixes #932