Signature Verifier about trillian HOT 2 CLOSED

Here's a general plan: @Martin2112 @AlCutter Please comment.

Digitally Signed should be it's own proto and contained go package for maximum software reuse.

• Signing and Hash algorithms should be separate enums inside DigitallySigned. This provides concept separation from tree hashing strategies and hashers for signing.
• DigitallySigned should be in it's own proto file and package.
• Signature package should sign generic Go objects with ObjectHash, and raw hashes. Type hashing / encoding should be moved to a supporting package or removed.
• KeyTransparency will discard it's signature package in favor of this one. google/keytransparency#489

from trillian.

• The signature proto has been separated into it's own proto package.
• The signature library has been separated into it's own package.
• The hashing functions for signing have been decoupled from TreeHasher
• Non-generic functions have been moved to a separate file, pending deletion.

from trillian.