Fix !closepoll command

Solves sqlite3 error "ProgrammingError: Incorrect number of bindings
supplied. The current statement uses 2, and there are 1 supplied."
Add multiple bot prefixes to poll announcement
Remove unnecessary success announcements

The commands' replies signals success to the user.  Additional message
is not needed and noisy
Convert voter's choice letter to upper case

Prevents "Error: That is not a choice for that poll"
Add ability to update a vote
Improved help messages
Fix success announcements
Fix error message in !closepoll command
Fix original poll announcement

Poll is now announced to the channel when created in PM
Merge pull request #1 from Erika-Mustermann/master

Various small fixes and features