configure database connection with environment variables (easier for dokku deployments)
Minor tweaks to WebmentionHandler.

* Do basic sanity checks *before* fetching the source document.
* If source is a valid post, also check for its referenced_url to be equal to target
Tweak Gemfile

* Move Capistrano to :development group
* Remove Invoker.
Apparently, we do need to require the capistrano libs.
Remove config/initializers/configuration.rb for now.

We'll do configuration checks later/somewhere sane.
Once again, don't require capistrano-*.

Sorry for the messy commits. :)
WTF is this shit. :(
Today, Rails hates me.
When deploying, ask for branch to deploy.
Clean up Gemfile. Again. :)
Update a bunch of dependencies.
Refactor all service classes.

* Awesome new Service base class!
* Logs each service to its own log file.
* Includes invocation information in exception reports.
* Much nicer invocation DSL.
More userful log output in UserPoller
Duh. Return proper service return value.
Fix log message wording.
Improve production.log through the magic of lograge.
Only show "follow" button when a user is known.
Bump version to r103
Fix user loading in PostFetcher.
Another fix for UserFetcher.