Rejecting requests for more than 96 descriptors

In our tor-dev@ discussion Karsten mentioned that we could request at most 96
descriptors at a time when polling by their fingerprints...

https://lists.torproject.org/pipermail/tor-dev/2013-June/005005.html

I've emailed him to clarify where this limitation comes from (the url length or
something within tor?), but in the meantime adding a check for this to our
get_sever_descriptors() method.