Add more examples

[jmitchell]

Currently the Sudoku solver is the only example.

Motivation

• Demos can help people realize the wide applicability of backtracking.
• Sudoku doesn't leverage the API's full flexibility. For example, values callback ignores its problem and unknown parameters.
• Optimizations like #3 will be better tested with a diverse collection of ready-to-go applications.

[jmitchell]

Started a password cracker example in e5d138326. Realizing the API and cracker needs some work:

• [x] Fix validity checking: currently example fails prematurely.
• [ ] Backtrex should support custom client state, perhaps by threading through callbacks.
• [ ] Support a set of optional of unknowns. Currently cracker must make an arbitrary, fixed length assumption.