Initial version of rate limiting on requests

First version of rate limiting on requests. Should fix issue #1877. I extended the `request.py` code, so this fix is generalized, and can be used for different situations, not only for Last.FM. It will only sleep the remaining time.

This fix does have side effects. For instance, the search page takes a lot longer, since every album image is a request to Last.FM. There are two options to fix this, a bit:

* Increase rate limit for 1 req/sec to 2-4 req/sec. Motivation for this is that `it worked before', without any rate limit
* Fix the search page to only request for the first items showing.

I will work on the second option, because I don't think it is hard to implement.