Fixing "set_table_name" deprecation warnings

Upgrading to rails 3.2 reveals a deprecation that is used heavily, and sprays out into our console profusely:

```
DEPRECATION WARNING: Calling set_table_name is deprecated. Please use `self.table_name = 'the_name'` instead. (called from acts_as_archive at /home/deploy/.rbenv/versions/1.9.3-p194/gemsets/accounts/bundler/gems/acts_as_archive-390fca44c084/lib/acts_as_archive.rb:132)
DEPRECATION WARNING: Calling set_table_name is deprecated. Please use `self.table_name = 'the_name'` instead. (called from <class:Archive> at (eval):2)
```

In my quick tests, these changes fix things. I made them in haste so some review might be in order.