added priority field

with three options (enforced in the Story model): High, Medium, and Low.
Made the field visible on all view pages, including
index pages, show a single story, edit/new story, edit multiple.
The priority defaults to "Medium".
Made a helper method,
ApplicationHelper:options_default(attribute, default_value),
which allows a choice to be made between a value,
e.g., one
in the database for an attribute, or a default.
We used this for Story.priority, so called
options_default(@story.priority, "Medium")
which means that if an instantiation of a Story has
a priority set in the database, use that value,
otherwise, present Mediium.
(We should use it for the Status field as well as the
Priority field!).
*** To run this, "rake db:migrate" needs to be run after
this code is downloaded ***