Add support for multiple exclusion policies, add Default group (enabled by default)

Changes/Additions:
- All properties without a group defined are now considered to have the group "Default" (consistent with Validator component)
- By default, the serializer only takes the group "Default" (consistent with Validator component) - possible to change by overriding `Serializer::createDefaultExclusionStrategy`
- Setting an empty group array enables only "Default" - to disable groups completely one can use `Serializer::setExclusionStrategy(null)`
- Added `Serializer::addExclusionStrategy` that adds a new one in the chain, but keeps the current ones (useful to add custom strategies safely)

Fixes #107