Add "WHERE-IN" clause support

[xinbenlv]

[kengz]

A solution to this problem: define a variable earlier that can be concatenated within the query like this

var set = '("foo, bar, baz '")';
new YQL('SELECT * FROM yahoo.finance.xchange WHERE pair IN '+ set);

You stringify the first layer of set so that it fits inside the YQL constructor. But when interpreted, the set value must be string. So stringify that second layer with the double quote.