RequestBuilder concept + CardDAV AddressBookMultiGet support

I am not entirely sure how you want to go about this, so I smashed a few things in for you perusal:

- I changed the namespace structure, this is more compatible/consistent with the existing client in sabre-dav, 
- I threw sabre-dav's client in, I get the impression you want to cover new ground with this client - but I think /some/ compatibility with the old client is probably worthwhile keeping around, I expect this will be significantly refactored regardless though, but either way, something half-working is better than nothing. Additionally, the above namespace change keeps the classname the same, which assuming we don't change too much, should let it swap out the old one.
- RequestBuilder - I thought about writing a class that implements the RequestInterface, but this seemed a bit hacky considering there are far fewer variables, the method is always the same, as are the headers (probably) and a few other methods that the interface specifies that would be redundant. Primarily, the goal is to make building the XML easier, so I think a builder is probably the right way to go, it's also quite flexible in what it defines, so quite extensible for other purposes too.
- PSR'n - I edited a few other things to just PSR-1/2 it up.