Add a vendor namespace

[keeguon]

As discussed here: https://plus.google.com/116975024782869852103/posts/DfGuV3s9SgU, Sebastien Schmidt proposed that I add a vendor namespace to avoid conflicts with other libraries/frameworks.

In short I'm not planning to do this. However if it's a real problem and people start reporting this I'll do it. Just for you guys to know if I do it, it'll probably be under a sub-namespace of MultiPass such as this:

<?

namespace MultiPass\Component\OAuth2;

// some code...

Let me know guys if the fact that it is not under a sub-namespace is such an issue so that I can take action regarding this matter.