Use get_FOO_display in choices-based FSM field logs

When displaying messages in the admin or creating LogEntry records,
use get_FOO_display method when reffering to FSM fields with defined choices.
Log FSM field's name into LogEntry records.

Put verbose_name of the FSM field into LogEntry records,
instead of writing just "state" regardless of the actual field name.
Merge pull request #9 from knaperek/master

Improve presentation of (uncommon) FSM fields