oops - backed out an extraneious newline
Added user stories as a set of goals for user interaction in the terminal
interface.

Reorganized main in controller.cpp so that the vector holding the initial
values (from input file or stdin) has only the needed lifetime, instead
of hanging around for the entire execution.

Added candidate handling, but it segfaults.  Setting it aside for now
to go on and revise it to use set or bitset.

dcc 11-Feb-2014
Changed underlying implmenetation of candidates to use std::set rather than
std::vector. Only a sanity check has been run (that is, we can solve using
the old backtracker, but setting the candidates prior to running the backtracker
does not blow up.  We should write code to print candidates for the whole grid.
We will want to have that available interactively too!