Added colorization to Scrolls logs

This commit solves the problem with hard to read logs.
It solves it by colorization of the output.

Scrolls now features an non-default mode flag.
Default value is production which is keeping the original behavior
of Scrolls logs.
Adding the mode development to Scrolls will make output on
development machines colorized, which is customizable as well.

New feature relies on the gem Rainbow.

To toggle colorization provide the following to the Scrolls.init()
method.

  :mode => "development",
  :colors => {:key_color => :green, :value_color => :blue}

There are some keys that you cannot color by default which is
keys provided by scrolls, they have been colored red and blue.

Rainbow: https://github.com/sickill/rainbow