Add datatype-forced sorting

[wshager]

Since dstore seems to follow RQL, I would like to see an option to force sort by datatype (e.g. number, boolean, date, etc.). Currently the prefix is escaped, and I think the correct place should be in the sort object: [{property:"id",datatype:"number",descending:false}].

Considering prefixes, I have an idea for an alternative notation: they could just be like functions, e.g. sort(number(id)). I already proposed this in persvr/rql#51.