Support weight in UVECTOR

If uvector->header.impl_flags has GRN_OBJ_WITH_WEIGHT flag, the
uvector has weight information.

If an uvector has weight information, it uses the following format:

    |grn_id1|weight1|grn_id2|weight2|...|
     uint    uint    uint    uint

Uvector without weight uses the following format:

    |grn_id1|grn_id2|...|
     uint    uint
Merge pull request #176 from groonga/support-weight-uvector

Support weight in UVECTOR