WattDepotClient methods should make samplingInterval optional

[cammoore]

Reported by rbrewer, Oct 30, 2011
The methods of WattDepotClient such as getEnergy currently require a samplingInterval parameter, even though in the REST API it is optional. This causes needless errors if a client wants to retrieve the energy data for a range shorter than the sampling interval.

These methods should all have a additional version that omits the samplingInterval parameter.

[rbrewer]

Duplicates #31