#150 store cache data in binary form.

[Kuang-che Wu]

This small patch let pisg use Storable to do caching
instead Data::Dumper. It stores cache data in binary
form instead of text.

Comparison with the original cache and the new one:
I run pisg with 4 channels x 1400 days log (one per
day), total 6.5M lines.
CPU time reduce from 132s to 87s.
Memory usage reduce from 23mb to 20mb.
Disk space from 209M to 154M.