Thumbnail image for each entry

[pmartin]

With this PR :

• Download the preview image for each entry (when wallabag has one)
• Generate a local thumbnail (small, 16 colors, PNG) so it can be displayed as quick as possible
• In the entries list, display the thumbnail of each entry (when there is one)
• When "deleting all local data", the images are deleted too

Current limitations :

• Images are downloaded synchronously, one at a time, when syncing from server => in the future, downloading several images in parallel should help speed things up (like it did for EPUB files)
• Only supports PNG et JPEG/JPG images ; and detection is based on the extension in the URL :-(

After a few hours of work and with these limitations, it's not perfect, but it seems to be working, so I'm commiting (and I'll probably merge soon) if someone wants to test (and I'm already using this on my own TL3 ^^ ) - fix #20 (even if some changes / enhancements might come in the future, on other PRs)

Oh and here's a quick screenshot (on a page where all entries have a thumbnail) :