Accept both String and Class instances as an ignored class.

If we accept actual Classes, developers can avoid typo-related
bugs if they so desire, e.g.:

```ruby
configuration.ignore_classes << "NotFoundEror"
 # => didn't catch a typo, sadface

configuration.ignore_classes << NotFoundEror
 # => Ruby will complain it can't find a constant
```
Merge pull request #119 from ecin/allow-non-string-ignored-classes

Accept both String and Class instances as an ignored class.