Fix '-t in args' check

fixes #77
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
Allow blank panes with options

fixes #76
Create spaces test dir if necessary
Merge remote-tracking branch 'ThiefMaster/fix-dash-t-command' #77
Merge remote-tracking branch 'ThiefMaster/shell-command-optional' #76
Merge remote-tracking branch 'ThiefMaster/startdir-spaces' #73