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