add partial sorting

This adds sorting, but I haven't figured out how to sort on foreign
columns. I am calling this on hold for now, and will probably try using
AJAX or a gem.

http://railscasts.com/episodes/228-sortable-table-columns
BooksController:
helper_method :sort_column, :sort_direction
and add sort_column and sort_direction private methods

app/helpers/application_helper.rb
sortable method

book index view:
sortable "title"
sortable "author_id" - this is incorrect
sortable "isbn10"
sortable "isbn13"

author index view:
sortable "title"
sortable "given_name" - this is incorrect

app/assets/stylesheets/application.css
add asc and desc gifs,
some stuff I don't understand

put images in public/images/
make sort start to work

pull ou the fields that I have not figured out how to sort.
This does not have complete coverage.
The sort problem is: we are using an ActiveRecord sort (Book.order) -
which doesn't let us sort by foreign fields. To do this, we'll either
need a join, or to sort again *after* pulling in from ActiveRecord.
Neither solution is likely to be pretty, and I haven't figured how to
factor it out.
I think this is another reason we might wish to roll-our-own ratings -
have average rating be a Book field.
Merge branch 'sortable' of https://github.com/nbirnel/Library-Application into nbirnel-sortable

Conflicts:
	app/controllers/books_controller.rb
	spec/features/books_spec.rb
complete test coverage for basic sorting

pull out unused code
Merge branch 'nbirnel-sortable'
Merge branch 'master' of github.com:Rails-Summer-Project-Noah-and-Matt/Library-Application