Add method to set experiment context

[yalisassoon]

When our users run A/B tests, they typically want to record that that A/B test is being run in Snowplow.

Currently our users do this manually and as part of that will typically define their own experiment context or event.

The idea behind this ticket is that we pre-define a schema and method(s) for setting the experiment.

The "experiment" schema should include the following data points:

• experimentName
• experimentId
• variationName
• variationId

I think it would make sense to have two methods:

1. One that sets the experiment context. From this moment onwards, the experiment context would be sent with every event recorded.
2. One that "unsets" the experiment context. This would stop the context being sent.