Added Xmatch service (fixes issue #49)

Real non-remote tests are still missing, will add them later today. Opening a PR already anyway to get some feedback from you :-)
Fixtures are never really needed, they can always be avoided. In this case it just saves me writing ``xmatch = XMatch()`` at the beginning of every test function where I use an `XMatch` instance. It's not much, but I have to say that I *love* py.test fixtures, so I maybe overuse them sometimes ;-) I like it if test functions are as simple as possible and mostly just contain a sequence of `assert` statements. But yes, maybe I'm exaggerating here a bit.