Further work on clarifying encoding to facillitate Python 3.x compatibility + addressing non-deterministic ordering of dataset attributes
* Modified the dods and das responses to encode as ASCII before yielding results
* Modified pydap.handlers.dap to decode the dds response as ASCII
* Modified pydap.responses.das to order keys before iterating over them
* Clarified the `six` module dependency to require a version that has urllib support