- add support for encoding NS (list of numbers)
- add EncodeAttributeValues
Conditions are more clear if we specify the operator first and then the
parameters.
Some work to manage range conditions (may move some of the stuff back into
conditions.go later on)
- Added table (*TableInstance) to QueryRequest so that we can get key
  definitions (and the DB context)
- Additional QueryRequest methods to manage range conditions
- Added implementation of "query" command with index and range