Add tweeted_poem
rake db:migrate
Belongs to poem
Add index for poem_id of tweeted_poems
Add code to check for tweeted poem

This code simply checks for a previously tweeted poem and increment the
count if it exists.  Future implementation could make sure that the
tweeted poem hasn't been tweeted in at least a certain time frame.

Also set default count of tweeted_poem to 0 to avoid nil.