implement structure for events

[A554551N]

Events are time-limited activities that allow users to receive special rewards for participating. Events will typically have:

• Some specific commands (they may be extensions of regular commands like .lure)
• A specific event currency that should be tracked (possibly with a user_event_currency field on the Users table)
• Easily turned on and off at the beginning or end of an event period (we should have this automated if possible)
• Commands that are event specific should be implemented in src/cogs/cog_event (should there be additional src/cogs/cogevent)?

[A554551N]

began work on implementation of event structure. Skeleton is in place, will work with @Zombpossum to determine what other elements need to be attached (and to receive list of event texts)

[A554551N]

implemented as part of release-0.3.0