Allow ActiveRecordViews to load ERB files

Partially fixes: #6 

First it checks for a matching `.sql` file before looking for a `.erb`

Could potentially use some work to make partial rendering a thing without having to go:

``` erb
<%= File.read '/path/to/_partial.erb' %>
```
I think this would read better as something like:

```ruby
      def is_view(sql = nil)
        sql_path = ActiveRecordViews.find_sql_file(self.name.underscore)
        ActiveRecordViews.register_for_reload self, sql_path

        sql = if sql_path.end_with?('.erb')
          ERB.new(File.read(sql_path)).result
        else
          File.read(sql_path)
        end

        unless ActiveRecordViews::Extension.currently_migrating?
          ActiveRecordViews.create_view self.connection, self.table_name, sql
        end
      end
```