Don't put using namespace in the headerfile

It caused compile failure on WIN32 due to ambiguity for
symbols.

Change-Id: Ica038cf0ee1919e9257646694aad03ff3e206e20
Reviewed-on: http://review.couchbase.org/31207
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>