AU: Payload Signer class
This class can take a private key and sign a blob of data. The API is
amenable to the upcoming change to delta_diff_generator that will use
it.
Also, minor change to the protobuf to support signatures.
TEST=unittests
BUG=5662
Review URL: http://codereview.chromium.org/3173032
diff --git a/unittest_key.pem b/unittest_key.pem
new file mode 100644
index 0000000..f3867e4
--- /dev/null
+++ b/unittest_key.pem
@@ -0,0 +1,15 @@
+-----BEGIN RSA PRIVATE KEY-----
+MIICXQIBAAKBgQDh27889vgney5Fmg89ZR80CFAhBWfj/33/6uRfWRs9tNP0kKpy
+Iml8Q4s6acPyp4VUlmfcayVKIkoNYhV+D9oRtNGmjosPCTRmupQ+55z0/FFHjtT5
+4sJShFvBcCofygvHeYqrzgpZdB8z1z/zhAfJc43hs2jFcr82UvkQ8XD3xwIDAQAB
+AoGAeiVwqFTcYeXBYYzu3wI4PNieFS2qQOjkyKnM9i/nhpir5GD0fSNVLwoMWvvm
+J+4nMlbhNpiPoycmK1S3UOTbxqFgw5b0DSUMiiGBdPIc7X0hPVIeR4I40g81fFl0
+3EyvZxlZexzfeTISbp+gpa+KW1bYoxY7e9TWCQDP+cuB34ECQQD5uJXzjU5AKTpA
+hgE1u2WetuGsnl95S96cBwuCiULAcbdFXQYip57eODfoNI2QDGyhdhcr3OBzhbR5
+ScK1qIXBAkEA54mPrA+65eEtv2fD4Iqt1qcvuVll/fSIdq6yQJEnzCUEsT7Ink0C
+WvAAHYlBISkc8onIagPowCQfGbVangQvhwJBAJ438HIOfpyyQmEtRkj4AausrZGE
+CnO8uT9cS1OaifuKURcWmFOOpl6fefSaj3LMHGu0eXvmByPKfA04ya/1JUECQQCX
+FQAW+jyue/zqBL+f6V39zyIpA9i1mbbiGqRd1Vnur8kcDyfBg+ahiDHLFCDXjohB
+Cv8njl114xwYHmp+6aRJAkAtumVpfP3KIAg7q7pXT5W1yTTqrzN/xAn4AQhnaXuh
+6Kt1dgWF7IQUEzoMKHBgciuwh2KjIxOx1/sMfLj3nz3H
+-----END RSA PRIVATE KEY-----