adding groupId to context.groupId to support Totango

Just got off a call with Totango and @ianstormtaylor. Totango needs `sdr_o` on server-side calls which is their account level `groupId` on `identify`, `track`, `page`, and `screen` calls. Ian and I decided to put it in `context.groupId` until we support some kind of state or persistence server-side.