Add a "rake console" command

This is useful for hand debugging, inspired by Ernie Miller's
http://erniemiller.org/2014/02/05/7-lines-every-gems-rakefile-should-have/

A consequence of this is to make pry a project requirement, so it has
been moved in the Gemfile.

Another consequence of this was my use of require_relative, and that
meant that lib/quiz_maker needed a tweak to use it too.
Use require_relative in bin/populate

This simplifies the code a bit.
Use dotenv gem to put the database URL in a file

This avoids the repetition of the database URL in the code. See
https://github.com/bkeepers/dotenv for more information.