Model and relations for following #46

Seed data with follow relationships
book model, views, seeds #46

followed_by_user method, books index and show adding followed
placeholders
Seeds file updated so that there follows contain at least a rating or
review
adds ability to set per-book preferences #46

Form on book#show with prefs
partial for follow prefs
change routing
add follows_controller
follow books checkbox #46

Javascript to set/unset follow on a per-book basis
controller
set @follow object if signed in on book#show
follows controller create and destroy, params list typo
after_save callback on follow model to delete follows with no rating or
review attribute
checkbox to set follow on books#index
form to set follow attributes on books#show
Test for Follow model #46

Adds Follow factory
adds model test for Follow.rb after_save callback