Enforce execution of field update operations is in alphabetic order

[tatu-at-datastax]

(from #132)

Currently update operations on fields are executed in the order specified by input document, so for example:

{"$set": {"foo" : 1, "bar" : 2}}

would first set value of "foo" and then "bar". Specification mandates ordering to be the opposite.
The most obvious change resulting from this is that in case of adding (setting) new properties, insertion order is changed; for most other aspects it should not significantly change behavior.