Add subcommand to check storage consistency

Currently only checks for overlapping entries, more checks will be added with
time.
Release 0.12.0
Bump copyright years
Fix run with currently running task
Rearrange Event.start for better error messages
[Tr] Minor doc duplicate word fix
[Tr] Add alternative text to README images
Fix ledger output when task is running

We were breaking out of the output when hitting a running task, as opposed to
just skipping the running task.
Fix stop --amend running check
Bump copyright years in setup.py
Use /usr/bin/env to start Python

This breaks the common case for me, but people who use virtualenv moaned enough
to make me switch ;)