Added ActivationStrategyProvider with default implementation

- Default implementation uses ServiceLoader to discover strategies
- Added activationStrategyProvider to FeatureManagerBuilder
- Resolved Activation Strategies may be empty in DefaultFeatureManager
- Added static ID to GradualActivationStrategy
- Added test case for when the activation strategy is not found for a
  FeatureState
Merge pull request #72 from jkrshw/activation

#71 Added ActivationStrategyProvider with default implementation