Define method for GaStartDate to get first date of when data is available in the reports

[jdeboer]

Consider the class signature for this method as a gaView or gaProfileId, with perhaps an optional metric as the second argument with a default of "ga:sessions".

Could also be done similarly for GaEndDate and GaDateRange.

Consider using the creation date property of the gaView object as a starting point for lookup.

[jdeboer]

Implemented methods for generic functions as outlined in this issue as part of the following commit for objects of class gaView: 59d64dd