[cffLib] Packing charset with only .notdef glyph fails

Fixes https://github.com/behdad/fonttools/issues/136
[merge] Fails if 'GSUB' table not present

Fixes https://github.com/behdad/fonttools/issues/135

Or does it?
[glyf] Align glyphs at word boundaries, not long

Saves two bytes per glyph on average.  Or as Roozbeh reports,
70kb in DroidSansFallbackFull.ttf.
[cmap] Remove bogus use of font.lazy

Ouch!
Make glyf-table glyph loading lazy by default

When I added the font.lazy setting, I made glyf table non-lazy
by default.  This is helpful to users who typically access glyphs
like:

	glyf_table.glyphs[glyfname]

instead of the correct way:

	glyf_table[glyfname]

and also forget to call expand() on the glyph.  However, this
significantly slows down most scripts that load the font without
lazy=True...  As such, add a third mode to laziness.  By default
lazy=None and does NOT expand glyphs.  If lazy=False is passed
in, all glyphs are loaded.

I hope this is an acceptable middle ground and not too confusing.