Remove support for overwriting a table twice in SFNTWriter

Was slightly broken already.  Remove it as there's no reason
to allow that.
Unbreak Python3

Fixes https://github.com/behdad/fonttools/issues/197
[TSI1] Use bytes literal
[TSI1] use tobytes()
Merge pull request #198 from moyogo/py3tsi1

[TSI3] use bytes literal and tobytes()
Read documentation.html as webpage

pro-tip: use rawgit.com to read HTML
Won't handle getting slammed on HN though ;)
Merge pull request #202 from tomByrer/patch-1

Read documentation.html as webpage
[subset] Update maxp values when dropping hinting

Fixes https://github.com/behdad/fonttools/issues/201
[subset] If DSIG is to be retained, drop all signatures

Now DSIG can be retained by passing --drop-tables-=DSIG.
The embedded signatures will be dropped but empty table
retained.

Fixes https://github.com/behdad/fonttools/issues/145