Do not tag clients with nil tag

With no selected tag (e.g. during startup, as long as no client with a
rule has been processed), cur_tag would be nil and then the match for
exclusive would be ~= true.
Merge pull request #24 from blueyed/match_client-dont-tag-with-nil

Do not tag clients with nil tag
Add settings.force_odd_as_intrusive option
Improve documentation
Add Awesome 3.5.3 support
Typo in xephyr.sh comment
Merge pull request #29 from ch3sh1r/patch-1

Typo in xephyr.sh comment
Fix typo in README
Merge pull request #30 from SammysHP/patch-1

Fix typo in README
Fixed error in README. First value is instance, second class.
Merge pull request #32 from junipergold/master

Fixed error in README. First value is instance, second class.
Restore shifty-like functionality to exec_once

exec_once always runs if the command isn't already running, regardless
of whether it's been run before in this session.
remove unused code
Merge pull request #35 from gotno/exec_once

Replace the old exec_once logic with a more useful one
Add support for fallback tags
Fix issues with fallback tags
Add support for locked tags
Add max_clients support
Merge the fallback branch
Update README.md