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