Cleanups.

I made 4 cleanups in 4 commits, so it's easy to pick the ones you like. (Or grab all.) : )

- Fixed grammar in install.rb
- Removed ClassMethods/InstanceMethods modules from OpenIdStoreActiveRecord, because it's not necessary in your case
- Renamed column `issued` to `issued_at`, to reflect (as per convention) that it's a datetime.
- Remove tasks dir, which isn't used.