Correct namespace for CTag property

The 'getctag' property should be in the http://calendarserver.org/ns/
namespace instead of DAV:
Merge remote-tracking branch 'upstream/master' into ctag
Return the display name from a DisplayNameResource.
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Merge branch 'master' into ctag
Merge pull request #31 from dachristensen/ctag

Correct namespace for getctag property
Merge pull request #30 from dachristensen/displayname

Return the display name from a DisplayNameResource.