Added compatibility with Polylang.

Here are my changes to add compatibility with Polylang. I have added a language fields to the subscriber-table in the database, which keeps track of the language the used had chosen when he subscribed. In case the site does not use Polylang, the language field is simply left empty. Other than that, all Polylang-dependent parts of the code are wrapped in ifs, so that it is only run when Polylang is present. Finally, I have changed the subscriber list to a table, which looks nicer.

I have tested the code on my (new) site at www.acrux.dk, which runs Polylang, and also on a clean Wordpress installation without Polylang.

Feel free to ask if you have any questions or discover any bugs :-)