Simplify avatar system

[mvz]

Publify has an avatar plugin system, but part of it is implemented in publify_core, and part in publify. Also, there is only one plugin.

Proposal:

1. Move avatar plugin system and gravatar plugin into publify_core.
2. Switch to libravatar, implemented by the Vishnu gem. This is a superset of gravatar since it will fall back to gravatar if no libravatar avatar is found (see the libravatar api documentation).
3. Remove the plugin system in favor of some libravatar settings (on/off, fallback picture type)