Deprecate numeric argument to for 'default_format'

It will warn when the application will pass numeric representation of
the flags to 'default_format'.

Old:

    Couchbase.connect(:default_format => Couchbase::Bucket::FMT_MARSHAL)

New:

    Couchbase.connect(:default_format => :marshal)
    Couchbase.connect(:transcoder => Couchbase::Transcoder::Marshal)

Change-Id: Ic25c6e584820b39c89253ac63e57f5923b867918
Reviewed-on: http://review.couchbase.org/25940
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>