multiget abstract base class

Refactored common code from caldav/MultiGetReport and
carddav/AddressBookMultiGetReport into common abstract base class. This
resolves an issue where the address book multi get was not properly
decoding the HREF before looking up the resource.
Fixed mult-get base class

Previous commit accidentally left AddressBookMultiGetReport extending
from the caldav MultiGetReport instead of AbstractMultiGetReport.
Merge pull request #15 from dachristensen/master

multiget abstract base class