Added class DigestSha256Signature. Refs #1851.
 tests: In test-encode-decode-data, show a DigestSha256Signature signature. Refs #1851.
 encoding: Added encode/decode support for TLV signature type DigestSha256. Refs #1851.
KeyLocator: Added static method canGetFromSignature. Refs #1851.
 security: In ConfigPolicyManager verification, fail if the signature type doesn't have a KeyLocator. Refs #1851.
 security: In PolicyManager.verifySignature, support DigestSha256Signature. Refs #1851.
security: In PublicKey.getDigest, we don't have to copy the hash to return the Blob.
 security: In selfVerifyPolicyManager.verify, only get the public key if canGetFromSignature. Refs #1851.
KeyChain: Added signWithSha256 for Data and Interest. Refs #1851.
security: Need to require "crypto" to use hash. Refs #1851.
unit-tests: In TestDataMethods, added VerifyDigestSha256. Refs #1851.
Data: In setSignature, always call clone if the signature is not null, so that it works with DigestSha256Signature too. Refs #1851.
security: Need to require Blob.
Data (internal): In getSignatureOrMetaInfoKeyLocator, need to check KeyLocator.canGetFromSignature.
ndn.js: Rebuild to get support for DigestSha256Signature. Refs #1851.
CHANGELOG: Added DigestSha256Signature and related info. Refs #1851.
Merge branch 'issue/1851-implement-DigestSha256'. Refs #1851.
http://redmine.named-data.net/issues/1851