FIX: hdfstore queries of the form where=[('date', '>=', datetime(2013,1,...

    FIX: hdfstore queries of the form where=[('date', '>=', datetime(2013,1,1)), ('date', '<=', datetime(2014,1,1))] were broken
    - modified Expr.parse_back_compat to check for tuples, in w, and unpack into w, op, value
    - modified Expr.__init__ to modify the where list/tuple with the parsed result