Feature/24 avatar browser cache

On my quest to making small pull requests, I'm making this pull request which contain one **tiny** step to supporting remote avatars.

Features:
* Remove avatar cache on disk
* Use browser cache instead with cache timeout set to 10 minutes.
* Enable to server remote avatars __if__ you set up "convos:host2convos" in Redis manually
* Failing TODO tests in t/avatar.t

Feature related to remote avatars:
* Setting "convos:frontend:url" in Redis which again will be used in Mojo::IRC->name.

So the idea is to fetch Mojo::IRC->name, using ```/whois <nick>```. but this needs to be done without echo back to the web client. Any better idea is more than welcome.

I also wonder if Mojo::IRC->name should be set to "Convos $url" instead of just "$url". Could be nice to see when doing manual "/whois".