Signature: Added SignatureSha256WithEcdsa.

Change-Id: I73a0bdf6b6d966634daa6f121b65175fc82e6fd5
Refs: #1650
changes: Add 'Changes' section to track changes since previous versions of the spec

This commit also includes updated HTML CSS.

Change-Id: I0f45e3fba25587f97cbb63332d457baacb515f40
signature: Rename KeyLocatorDigest into KeyDigest

Change-Id: Ide8cace988e95269ec057786fc80db8036984c7f
signature: Defining KeyLocator as an optional field in generic SignatureInfo

Change-Id: I2e2850bdf5b1795c07aea208fe8c80c8b6df509f
Small fixes and clarifications

Change-Id: Ieb5669d21b9bb53c4c749271f25792e394130b26