Initial implementation of Manifest HTTP API

Push, pull and delete of manifest files in the registry have been implemented on top of the storage services. Basic workflows, including reporting of missing manifests are tested, including various proposed response codes. Common testing functionality has been collected into shared methods. A test suite may be emerging but it might better to capture more edge cases (such as resumable upload, range requests, etc.) before we commit to a full approach.

To support clearer test cases and simpler handler methods, an application aware urlBuilder has been added. We may want to export the functionality for use in the client, which could allow us to abstract away from gorilla/mux.

A few error codes have been added to fill in error conditions missing from the proposal. Some use cases have identified some problems with the approach to error reporting that requires more work to reconcile. To resolve this, the mapping of Go errors into error types needs to pulled out of the handlers and into the application. We also need to move to type-based errors, with rich information, rather than value-based errors. ErrorHandlers will probably replace the http.Handlers to make this work correctly.

To provide rich error reporting during manifest pushes, the storage layers verifyManifest stage has been modified to provide the necessary granularity. Along with this comes with a partial shift to explicit error types, which represents a small move in larger refactoring of error handling. Signature methods from libtrust have been added to the various Manifest types to clean up the verification code.

A primitive deletion implementation for manifests has been added. It only deletes the manifest file and doesn't attempt to add some of the richer features request, such as layer cleanup.

Unrelated to the above, the "length" parameter has been migrated to "size" for completing layer uploads. This change should have gone out before but these diffs ending up being coupled with the parameter name change due to updates to the layer unit tests.