Added support for recurly.js signature generation

Signature Generation for Recurly.js
-------------------------------------

A special signature string is needed for the BuildSubscriptionForm helper method in the recurly.js library. To obtain a signature on the server to pass to the page, simply use:
	
	String signature = <client>.getRecurlySignature(String jsPrivateKey);

If you wish to specify extra parameters (such as account id and subscription code), build an ArrayList<String> with url-encoded key=value string pairs. For example:

        List<String> extraParams = new ArrayList<String>();
        extraParams.add(String.format("%s=%s", "subscription%5Bplan_code%5D", "testplan1"));
        extraParams.add(String.format("%s=%s", "account%5Baccount_code%5D", "123abc"));
        String signature = <client>.getRecurlySignature(String jsPrivateKey, extraParams);
	
Refer to the [Recurly.js Signature Generation documentation](http://docs.recurly.com/api/recurlyjs/signatures) for more information on the format for building parameters.