Issue #83: Make DocumentFilterManager public
Issue #83: Make DocumentFilterManager public

Motivation
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Make DocumentFilterManager public so that domain POCO's do not have to
have attributes explicilty defined.

Modifications
-------------
Change scope of DocumentFilterManager from internal to public.

couchbaselabs/Linq2Couchbase,brantburnett/Linq2Couchbase,drGarbinsky/Linq2Couchbase