Log Message:
Update to 0.15:

Devil's Pie "Shogun's Decapitator" 0.15
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* New matcher: window class (Jonas Linde)
* New actions: focus (JL), center (JL), print (Ryan Koppenhaver), close (RK).
* Fix test suite (RK)
* Don't segfault on invalid s-exp (Christian Krause)
* Never return NULL from a matcher
* Wrap X call when removing decorations with error handling