Support all CRUD operations

[voxpelli]

The main blocker for a 1.0.0 release I think is supporting full CRUD operations. To be a good citizen with in the IndieWeb community one should respect peoples changes to and removal of posts and the current code of this app only accepts the very first initial creation ping and considers all subsequent pings as weird duplicates and that has to change.

Related IndieWeb wiki pages: http://indiewebcamp.com/CRUD

• update of entry and mention on repeated ping
• removal of entry and mention on new ping from same source
• removal of entry (and probably mentions) when source is tombstoned
• removal of entry and mention when source is completely gone (eg. 410)
• sync implementation with the flow described in the wiki
• add any meaningful documentation of this implementation to the wiki

[voxpelli]

First step on the way to full CRUD-support added. Updating should be more or less fully supported.

[voxpelli]

Should also look into supporting tombstoning of entries, as that's pretty much how removals of mentions are handled currently by the code, by tombstoning them: https://indiewebcamp.com/deleted#Tombstoning