add ECDSA example

examples/ecdsa.js

@@ -0,0 +1,21 @@
+var ECDSA = require('../lib/ecdsa');
+var Keypair = require('../lib/keypair');
+var Hash = require('../lib/hash');
+
+//ECDSA is the signature algorithm used in bitcoin
+
+//start with a keypair that you will use for signing
+var keypair = Keypair().fromRandom();
+
+//a message to be signed (normally you would have the hash of a transaction)
+var messagebuf = new Buffer('This is a message I would like to sign');
+
+//calculate a 32 byte hash for use in ECDSA. one way to do that is sha256.
+var hashbuf = Hash.sha256(messagebuf);
+
+var sig = ECDSA.sign(hashbuf, keypair);
+
+//Anyone with the public key can verify
+var pubkey = keypair.pubkey;
+console.log('Valid signature? ' + ECDSA.verify(hashbuf, sig, pubkey));
+