Migrations generated from resources file cause name collisions

[ShaunaGordon]

It seems wn:resources doesn't check to see if a create file for the resource's table already exists. When combined with the automatic migration attempt, this results in a cannot redeclare class error, which kills the migration.

Steps to reproduce:

1. Create a resource using the single wn:resource command.
2. Create resources (including the previously-created resource) with a file and the wn:resources command.

Expected actions would probably be to look for that class name and one of:

1. Adjust the name of the incoming generated class so as to not collide
2. Overwrite the found class file (probably not recommended in most cases, but may be useful with the --force switch)
3. Not create a migration file for that resource (or better yet, create an update migration with the changes from the existing schema)