add base64 support

The project I'm working on involves an application that accepts raw file data encoded in base64. Other xmlrpc libraries I've used such as ruby's ([from its standard library](https://github.com/ruby/ruby/blob/4c2304f0004e9f1784540f3d36976aad9eab1f68/lib/xmlrpc/parser.rb#L443)) have support for base64. This PR adds support for it to this library.

I've used it in the project I'm working on and works well, it's pretty much just like the string tag, but I needed to create a type synonym in order to type check against it, so base64 is sent like:

``` go
client.Call("some_func", xmlrpc.Base64(base64data), &result)
```