Add Manifest support according to the W3 standard

[austdev]

Add new methods for working with Manifest extension:

func (ctx *SigningContext) CreateSignature(id string) *etree.Element
func (ctx *SigningContext) AddManifestRef(sig *etree.Element, name string, hash_id crypto.Hash, digest []byte) error
func (ctx *SigningContext) SignManifest(sig *etree.Element) (*etree.Element, error)

func (ctx *ValidationContext) ValidateManifest(sig *etree.Element) (*types.Manifest, error)
func (ctx *ValidationContext) VerifyReference(ref *types.Reference, data []byte) error
func (ctx *ValidationContext) DecodeRef(ref *types.Reference) (crypto.Hash, []byte, error)

An original API is preserved

[russellhaering]

Sorry for the silence on this, it’s next on my list for review.

[austdev]

It's Ok. I tried to resolve conflicts quickly, but result was broken with some compiler errors. So, I need more time for this. Probably, would it be better to split this request on smaller parts? Or exclude some commits? Please, let me know.