Added solution to problem 243.

Solution involved enumerating numbers with small prime factors (below a
certain limit).
Solved problem 80
Added solver for sudoku puzzles for problem 96 (but not the plumbing)
Added solution to problem 205
Added solution to problem 104
Added solution to problem 68
Solved problem 83.

Generalises to problem 81 and should solve problem 82 too but seems to give a
different result.
Generalised solution to problems 81-83.
Added note on bug in solution to problem 83
Merge branch 'problem-83'
Added solution to problem 91
Solved problem 77
Solved problem 108 - solution will work for problem 110 too but need bignums,
so reimplementing it in Perl.
Added solution to problem 110 - same algorithm as 108 but with bignums
Solved problems 116 and 117
Solved problem 113
Solved problem 173
Solved problem 107
Added brute-force solution to problem 214
Added solution to problem 123