v2.0 milestone

### Breaking 

**Important:** This PR brings changes that are breaking the backwards compatibility. While maintaining the backward compatibility is the priority following breaking changes are important to improve the integrity and clarity of serialized API Blueprint AST. 

The breaking changes are as follows:

1. Use array instead of hash for metadata, headers and parameters - see #1 
2. Remove headers entry from resource and actions (compliance with 1B) - see #2 
3. Encapsulate parameter enum values in object - see #3