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/oauth-sign/README.md

@@ -0,0 +1,11 @@
+oauth-sign
+==========
+
+OAuth 1 signing. Formerly a vendor lib in mikeal/request, now a standalone module. 
+
+## Supported Method Signatures
+
+- HMAC-SHA1
+- HMAC-SHA256
+- RSA-SHA1
+- PLAINTEXT