v0.9.0 - changes to exposure logging on Layers

Previously exposures for Layers happen when you call getLayer(), but when you have multiple experiments running in parallel in the same Layer, this could be exposing more users than the actual number of users who were actually exposed to some of the experiments, so with this release we have moved the exposure logic to be happening when Layer.get() method is called, so that a user is only exposed to an experiment when the parameter for that experiment is fetched.