New $size operator and extended dot operator subdocument search

- implemented MongoDB $size operator
- extended dot operator so it can match properties of subdocument. For example, for the document { childrens: [ { name: "Huey", age: 3 }, { name: "Dewey", age: 7 }, { name: "Louie", age: 12 } ] }, it is possible to compare every the age in every subdocument with childrens.age notation