Update README* files, add 'small_object_divider' check to riak_cs_lfs_utils.erl

Update the README.multi-backend.txt file with a pointer to the new
README.fs2-backend.txt file.

Add a new application environment variable, small_object_divider, checked by
riak_cs_lfs_utils:calculate_bucket_class/1 when new files are uploaded.

        The value of this setting can be adjusted to fit your local
        environment.  The units of this configuration option is
        "bytes".  The default value is 128,000 bytes.

        As new files are uploaded to Riak CS, the
        'small_object_divider' setting is consulted to decide whether
        the blocks for that object should be stored in 'small' block
        storage (using a bucket prefix of <<"1b:">>) or in 'large'
        storage (using a bucket prefix of <<"2b:">>).  The choice of
        bucket prefix is detected by the multi backend in order to
        segregate block storage to the ELevelDB backend or FS2
        backend, respectively.