Verb field as enum rather than charfield + migrations

From https://github.com/justquick/django-activity-stream/issues/116

Thanks to  @JeromeParadis for the code! 
I wrote the migration to be non-destructive to your database. Also, it should migrate automatically your verbs to the new verb_id format. 

Only a little awkward for the settings of new verbs which will not start from 0 or 1 as expected but from N (your number of used verbs.)

Please review and try on test db first.

Vince