add actual content to a subject's page

+ test relationships between textbooks, subject, field
+ add method 'textbooks' to subject for easier access
generate fields controller
show field separately from subject

[Delivers #53290437]
append 'spec_helper' to the model's specs
load field's textbooks through ajax

+ gem 'database_cleaner'
+ move textbook html to partial
[Delivers #53298107]
show problems on the home page

[Delivers #53311425]
add 'guard-rspec' gem