Major changes. The methods to manipulate a calendar collection were in the DavStore, which didn't make sense. To be able to move the methods, a new class, HTTPConnectionManager, have been created so that information about the HTTP connection can be pass along.
If the eTag is empty, don't send the If-Match header
If the server (Google Calendar) is not sending the etag in the headers, we have to fetch it with a PROPFIND request
Merge pull request #18 from pascalrobert/master

Moving the calendar collection actions to the correct class.