Update link to spec in the ReadMe.
Update link to spec in the code comments.
Merge branch 'next'

* next: (28 commits)
  Refactor the manifest initialization process.
  Unset the manifest edited flag after saving it.
  Only create the .ro dir when creating a new bundle.
  Yield a newly created bundle locally for easier setup.
  Duplicate json structures before cleaning them.
  Add Manifest#write so it can write itself out.
  Add the Manifest#initialized? method.
  Tidy up empty bundle creation test.
  Allow direct editing of annotations via the manifest.
  Allow direct editing of aggregates via the manifest.
  Wire up the edited? methods for manifest entries.
  Use a new base class for manifest entries.
  Document the Provenance#retrieved_from(=) methods.
  Make sure Provenance#retrieved_from= uses Strings.
  Add support for ORE Proxies to Aggregates.
  Fixup the documentation of Aggregate#mediatype.
  Cache the gems used in the tests in Travis.
  Update the RO spec and homepage URIs in the gemspec.
  Tidy up the gemspec file.
  Add test-unit gem to the dependencies.
  ...