Allow REFER to non-SIP resources

TL;DR Demo: http://jsfiddle.net/MAd5U/

I decided to come back to this and make it work with minimal modifications to the Grammar. It allows an `absoluteURI` to be given as the value of a Refer-To header, without affecting other URI behaviors.

fixes #9

Usage:
-
  Send:
```javascript
    session.refer('http://www.ietf.org')
```
  Receive:
```javascript
    session.on('refer', function handleRefer (request) {
      var uri = request.parseHeader('refer-to').uri;
      if (!uri.scheme.match("^sips?$")) {
        // handle non-SIP referral
      } else {
        // handle SIP referral
      }
    });
```