Fix query interpolation

[ruiwen]

Thinking that interpolation might be more neatly done via str.replace() instead of string splitting along '?' characters. This patch allows for multiple interpolations as well, eg.

this.where("user.name === ? || user.age > ?", "john", 3, function(data) { 
    /* do stuff */ 
})

This patch also ensures that the callback is called at the end of the query