Implement MapQuest Elevation Service

API key added to either constructor or as MAPQUEST_API_KEY environment
variable. Request made via HTTP GET and returned as either error or
response parameter to callback. Ref #43
MapQuest library framework.
Parse results before returning in ElevationService.
Change MapQuest library to not require instanced objects.
Merge MapQuest ElevationService into develop.
Add getAirDensity to atmosphericObservation.