Use db migration script instead of rake mole:install

[Eric Pugh]

I just installed my Rails app on a new server, and because I didn't
install the plugin via ./script/plugin, the install.rb for Mole was
never run. Which meant I didn't have the mole database tables!

I added to my instructions:

6) Create the mole tables
ruby ./vendor/plugins/mole/install.rb

However, it would be much nicer if running rake mole:install generated
a database migration instead in my db/migrate directory. That way as
part of my initial rake migrate the mole tables would be put in!

Cheers,
Eric