Add imgur & twitter extractor

This should fix #13 & #14
Cleaner Proxy
Better mock on FetchItems test
Use Guzzle instead of raw cURL in rss validator
Add Tumblr extractor

Fix #15
Improve test on commands

cleanup twitter & rss validator test
Remove url from extractor

For the moment, I don't find a necessary interest.
So I prefer to remove it.
From what I imagine about extractor, they only use a different way to
retrieve the content. They have nothing to do with the url
Merge pull request #16 from j0k3r/extractor-chain

Add extractors
Add an init method on Proxy

To avoid bad behavior on consecutive calls
Merge pull request #18 from j0k3r/proxy-init

Add an init method on Proxy
Fix wrong place to reset content & url
Fix some alerts from Insight

https://insight.sensiolabs.com/projects/b3688b01-a0db-4325-a8ba-dcc59ab00fe4/analyses/36
Fix annotation