[subset] Don't recalculate "modified" timestamp

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

Fixes https://github.com/behdad/fonttools/issues/115
diff --git a/Lib/fontTools/subset.py b/Lib/fontTools/subset.py
index f99d430..f65a5a3 100644
--- a/Lib/fontTools/subset.py
+++ b/Lib/fontTools/subset.py
@@ -1824,6 +1824,7 @@
   notdef_outline = False # No need for notdef to have an outline really
   recommended_glyphs = False  # gid1, gid2, gid3 for TrueType
   recalc_bounds = False # Recalculate font bounding boxes
+  recalc_timestamp = False # Recalculate font modified timestamp
   canonical_order = False # Order tables as recommended
   flavor = None # May be 'woff'
 
@@ -2113,6 +2114,7 @@
                       allowVID=allowVID,
                       checkChecksums=checkChecksums,
                       recalcBBoxes=options.recalc_bounds,
+                      recalcTimestamp=options.recalc_timestamp,
                       lazy=lazy)
 
   # Hack: