Add unsigned 64 bit integer as supported datatype

This ticket tracks adding support for 64-bit unsigned numbers.

Arbitrary precision number support is tracked in SERVER-1393.

Original description:
Mongo doesn't support two useful (at least in my project) datatypes:

• unsigned 64 bit integer
• arbitrary precision (long) number

We need to search this fields with $gt, $lt etc, so a binary type is not a solution