Rackspace UK support

Added support for Rackspace UK customers.

When using my account name and API key I get upload errors

```
Thomas-Weltons-MacBook-Pro:ShakeYourMoneyMaker thomaswelton$ grunt cloudfiles
Running "cloudfiles:prod" (cloudfiles) task

Uploading into mentosmoneymakerlocal
>> Error: Invalid URI "/mentosmoneymakerlocal/somefolder/assets/stylesheets/compiled/template.css"

/Users/thomaswelton/Sites/ShakeYourMoneyMaker/node_modules/grunt-bower-task/node_modules/bower/node_modules/tmp/lib/tmp.js:219
    throw err;
          ^
TypeError: Cannot call method 'request' of undefined
    at Request.start (/Users/thomaswelton/Sites/ShakeYourMoneyMaker/node_modules/grunt-cloudfiles/node_modules/pkgcloud/node_modules/request/index.js:582:30)
    at Request.write (/Users/thomaswelton/Sites/ShakeYourMoneyMaker/node_modules/grunt-cloudfiles/node_modules/pkgcloud/node_modules/request/index.js:1144:28)
    at ondata (stream.js:38:26)
    at EventEmitter.emit (events.js:96:17)
    at Client.request.buffer.emit (/Users/thomaswelton/Sites/ShakeYourMoneyMaker/node_modules/grunt-cloudfiles/node_modules/pkgcloud/lib/pkgcloud/core/base/client.js:181:49)
    at BufferedStream.pipe.process.nextTick.self.size (/Users/thomaswelton/Sites/ShakeYourMoneyMaker/node_modules/grunt-cloudfiles/node_modules/pkgcloud/node_modules/morestreams/main.js:28:44)
    at Array.forEach (native)
    at BufferedStream.pipe.piped (/Users/thomaswelton/Sites/ShakeYourMoneyMaker/node_modules/grunt-cloudfiles/node_modules/pkgcloud/node_modules/morestreams/main.js:28:17)
    at process.startup.processNextTick.process._tickCallback (node.js:244:9)
```

This problem is mentioned here on the pkgcloud repo https://github.com/nodejitsu/pkgcloud/issues/72

I have made updates that allow us to overwrite the authUrl used by pkgcloud, and also updated the readme to reflect these changes.

There is now a new optional config parameter authUrl which can be specified in the Gruntfile which I am able to set to the correct value for use by UK customers, which is lon.identity.api.rackspacecloud.com