Adding exception class Sabre_DAV_Exception_ServiceUnavailable
fixing class name in doc
Merge pull request #307 from DeepDiver1975/1.7

Adding exception class Sabre_DAV_Exception_ServiceUnavailable to 1.7
Updated changelog
Merge branch '1.7' into 1.8

Conflicts:
	lib/Sabre/DAV/Exception/ServiceUnavailable.php