Updated README
Gemfile now lists gems for MySQL

It currently specifies the 'mysql2' gem for Ruby users. This might
change in the future.
Updated Gemfile.lock
Added .bundle directory to .gitignore
Removed MySQL gems; details inside!

On my dev. machine, Bundler cannot exclude MySQL no matter how hard I
try. I ran `bundle install --without mysql` and bundle spits-out a weird
config file that, when read by bundle configuration, does absolutely
nothing.

MySQL support might return in the future! I might add an issue for
this.