Fix StartDirectoryTest

It never checked if the start directory was properly set on the right
window. Now it does.
Add testcase for start_directory with a space
Fix start directory with a space
Fix session renaming with a space
Remove now-unnecessary import
Use pane_current_path instead of pane_start_path

tmux 1.9 removed the latter
Create spaces test dir if necessary
Merge pull request #73 from ThiefMaster/startdir-spaces

Fix errors with start_directory containing spaces