In order to make metadata schema definition language independent,
bpmetadata now processes metadata content using protobuf.

Known issues:
- BlueprintVariable.DefaultValue is changed from interface{} to string.
  This can cause error messages while running bpmetadata on existing
  blueprints. Solution: run bpmetadata with -f to force creation of
  proto compliant metadata.
- Go docs will most likely have superfluous types and methods not
  directly related to metadata. Solution: use protos as the source of
  truth for metadata definition and specs.