use to_param for more Rails-compatible controllers

[katafrakt]

Right now I have to use something like

    if @page.update_attributes(attributes)
      redirect_to page_path(@page.name)
    end

in controller that manages Gollum. It would be nice to be able to use Rails-like redirect_to @page.

This pull request adds this.

[coveralls]

Coverage increased (+0.02%) when pulling 4707010 on katafrakt:to_param into 9118e1d on nirnanaaa:master.

[nirnanaaa]

Thank you 😄