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 elfVerifyPolicyManager.verify, only get the public key if canGetFromSignature. Refs #1851.
KeyChain: Added signWithSha256 for Data and Interest. Refs #1851.
unit-tests: In TestDataMethods, added VerifyDigestSha256. Refs #1851.
 unit-tests: In TestInterestMethods, added TestInterestMethods. 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