Removes deprecation warnings in Rails >= 3.1.

Replaced `.set_table_name` with `self.table_name=` method to avoid depracation warnings in Rails >= 3.1.
Fixed `UniqueNameValidator#unique?` to check uniqueness only with other records (banyan_single_group_model_spec:12 was failing without this fix).
Adds possibility to configure Banyan gem using Banyan.configure.

Adds configuration setting `unique_names_for_category_groups`, set to true by default (will validate unique names for CategoryGroups).
Adds configuration setting for unique validation of Category name.
Merge pull request #14 from partnerpedia/rails-3.1

Removes deprecation warnings in Rails >= 3.1.