[StockStats] Added files for Stock Stats exercise.
[StockStats] Added Book#to_s method.
[StockStats] Added a call to Float function in Book#initialize.
[StockStats] Added Book#isbn and Book#price accessors.
[StockStats] Replaced Book#isbn and Book#price by an attr_reader call.
[StockStats] Added Book#price= method.
[StockStats] Replaced Book#price= method by attr_accessor call.
[StockStats] Added Book#price_in_cents method.
[StockStats] Added Book#price_in_cents= method.
[StockStats] Added CsvReader class, split Book class in its own file, and changed code to use CsvReader to read files given as arguments.
[Account] Added files of the exercise.
[Word Frequency] Added files of the exercise.
[Word Frequency] Replaced ugly loop code by something more appropriate.
Updated README.md