CBLUITableSource cleanup. Added CBLUICollectionSource.

CBLUICollectionSource written by Ewan Mcdougall. It's not compiled into
the framework by default, to keep the code size down, but apps can
compile it in. (It is included in the iOS demo app, to ensure that it
keeps compiling.)
Fixed exception handling REST _all_docs request

Recent regression from when I merged the public/private view classes.
There's a unit test that should have caught this, but it turned out that
CBLRouter_Tests.m was accidentally never being compiled.