Lazily-load MapReduce query results

[seancribbs]

Reference: https://gist.github.com/38eb017bf86d055d2e4e

Results from a wrapped MapReduce query should not be retrieved (or the job invoked) until needed. This would probably be achieved by delaying execution until an Enumerable method is called, e.g. first, each, etc. The query could also be explicitly executed through calling run.