Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/http-signature/lib/index.js

@@ -0,0 +1,29 @@
+// Copyright 2015 Joyent, Inc.
+
+var parser = require('./parser');
+var signer = require('./signer');
+var verify = require('./verify');
+var utils = require('./utils');
+
+
+
+///--- API
+
+module.exports = {
+
+  parse: parser.parseRequest,
+  parseRequest: parser.parseRequest,
+
+  sign: signer.signRequest,
+  signRequest: signer.signRequest,
+  createSigner: signer.createSigner,
+  isSigner: signer.isSigner,
+
+  sshKeyToPEM: utils.sshKeyToPEM,
+  sshKeyFingerprint: utils.fingerprint,
+  pemToRsaSSHKey: utils.pemToRsaSSHKey,
+
+  verify: verify.verifySignature,
+  verifySignature: verify.verifySignature,
+  verifyHMAC: verify.verifyHMAC
+};