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 :-)
Should also accept `astropy.table.Table`s here.  It may be necessary to write an additional function to extract coordinate columns from the `Table` - I don't know if that functionality exists in astropy but we can check the docs or ask the list.  If it does *not* exist, it is an important feature that should be added.