Get rid of lots of NotImplementedError’s in travis log
Add _with('wvars')
Prefix _Windows.{append,insert,pop} methods with underscore

There are no such methods in Vim.
Remove failing _index, use (self.number - 1) instead
Add missing winnr key to segments_dictionary
Add window_title tests
Respect the fact that vim.eval in Vim returns str instances
Correct _construct_result for Python-3
Use better type converted for python-3*

Converter already existed, just needed to make it used by getbufvar
Merge pull request #920 from ZyX-I/fix-not-implemented-tests

Improve vim tests
Set VIRTUAL_ENV_DISABLE_PROMPT to 1

Otherwise powerline prompt is being prefixed by `(virtualenv-name)` in zsh which 
looks rather ugly. In bash prompt is always overridden by PROMPT_COMMAND, thus 
there is no actual need to set this variable.
Merge branch 'disable-virtualenv-prompt' into develop