Exclude Starscream dependency from core functionality

[chrispomeroyhale]

Is your feature request related to a problem? Please describe.
In our app two of the metrics we monitor is app startup time and memory usage. In evaluating the performance impact of this SDK on our app, in our usage the SDK takes an average of 1.0MB of additional memory and increases our app startup time by an average of 0.6 seconds. Since we are only using over-the-air functionality for now, when we experimentally removed the Starscream dependency the memory burden of integrating the SDK dropped to 0.5MB.

Describe the solution you'd like
Can we only include dependencies that are required by the SDK compilation? This may require moving some files around or re-configuring how Cocoapods (and SPM?) subspecs are configured.