Incremental updates. Fixes #15

This is an attempt to use the ETag header and since parameter to make the polling more effecient.

Sorry about all the stupid commits (updating README, then revert, commenting GPG, then uncommenting again), not entirely used to working in a fork. :)

The changes aren't super pretty, but I wanted to keep as close to the original design as possible, which may have streched it a bit too far. I'm thinking we could introduce some kind of indexer or fetcher class to help keeping track of ETags and state, but I'd like your opinion first.