Refactored arrowhead exercise

Not sure about the raise string literals, since they push lines to
over 80 chars.
Refactored dinodex to raise an error

Raises error upon initialization if one of the input CSV files
does not match an accepted format.

Also refactored a few predicate methods to make sure that they
do return a boolean method, or nil in the case of matches_weight?
(indicating an error -- that user tried to search by a weight
that wasn't 'big' or 'small'. still trying to figure out how/when
to catch this error)
Refactored robot_name to use short methods
Refactored do_it.rb to use meaningful names