Progress bar when running rake db:seed
Integrate with RailsNlp

Steps:

1. rails g rails_nlp:install
2. rake db:migrate
3. add 'include RailsNlp' to Book model
Speed up db:seed by only saving a fraction of the content of the books.

Use probability to evenly discard lines throughout the book, in order to
reduce the size of the records.

A value of 0.05 will retain 5% of the content, which is the highest
proportion possible whilst keeping the database quick to analyse.
Perhaps for testing purposes this can be run overnight.
Add ruby profiler as development dependency.
reduce_to uses an integer, is now a percentage.