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 :-)
The docs say in http://cdsxmatch.u-strasbg.fr/xmatch/doc/API-calls.html under the headline *Retrieving the available VizieR tables*: "To get the list of the VizieR tables which are available in the service, …", that's why I decided to name it with "available" in it. But I can rename it to something else. Is `is_table_available` grammatically more correct than `is_available_table`? I think I prefer your suggestion with `is_table_available`. I'm against `is_valid_table` tho
ough, because validating is a process of checking the input itself, such as parsing the given `table_id` and checking if the syntax conforms certain rules. But this doesn't happen here, this method only checks if the table in the list of all available tables and that's it.

Now that was a long paragraph for one method name :)