Better header parse with custom WSSE sintax

Changed the FileTransfer.cs#parseHeaders function to work with the changes of the https://github.com/apache/cordova-wp8/pull/62 pull request.

The problem is that, with WSSE headers with a VALUE like this:

```
UsernameToken Username="foo@bar.net", PasswordDigest="ZTQXXXXXXXXXXXXXXXXXXXXXXXXXXXX0NTJlMGE2M2I0NGJiYWM4NA==", Nonce="XXXXXXXXXXXXXXXXXXXXXXXXXXXXA==", Created="2014-10-11T18:48:07.232Z"
```

The current version of the function, will return null because of a bad parse by DataContractJsonSerializer. The pull request I mentioned suggests change DataContractJsonSerializer by Newtonsoft.Json for a better serialize and performance.