Move Pubsubhub to a default plugin

[ArthurHoaro]

• Adds placeholders in feeds templates to specify which hub is used (render_feed hook).
• Retrieve the publisher lib with Composer.
• Notify the hub everytime a link is saved (save_link).
• Adds a default PHP fallback if cURL is not installed.
• Silently log exceptions.

TODO:

• Update plugin doc for the new placeholders

Fixes #459

[ArthurHoaro]

The dependency isn't compatible with PHP 5.3 (or at least their composer.json say so). Pushing to v0.9.0.

[virtualtam]

Additionally, the Pubsubhubbub publisher should be deployed through Composer - #613

[ArthurHoaro]

Yeah, that's what I did.

[Lucas-C]

FYI, there is dead code remaining here:
https://github.com/shaarli/Shaarli/blob/master/index.php#L234

[ArthurHoaro]

Thanks!