Allow disabling 'head' table modified-timestamp recalculation

Part of:
https://github.com/behdad/fonttools/issues/115
https://github.com/behdad/fonttools/issues/46
[subset] Don't recalculate "modified" timestamp

To recalculate (which was the default before), use --recalc-timestamp

Fixes https://github.com/behdad/fonttools/issues/115