convert addresses to lower case when storing

Some sources encode a checksum into the capitalization of the ethereum address, this causes exportPrivateKey to fail as the stored addresses are all lower case.

lib/keystore.js

@@ -396,7 +396,7 @@ KeyStore.prototype.exportPrivateKey = function (address, pwDerivedKey) {
     throw new Error("Incorrect derived key!");
   }
 
-  var address = strip0x(address);
+  var address = strip0x(address).toLowerCase();
   if (this.encPrivKeys[address] === undefined) {
     throw new Error('KeyStore.exportPrivateKey: Address not found in KeyStore');
   }