Improve OpenType/TrueType support

[simoncozens]

This PR:

• Adds support to the ttf parser for OpenType fonts (same content, different magic number)
• Allows the parser to unpack the maxp, hhea, OS/2, post, cmap and DSIG tables.

[simoncozens]

Good idea; I will get to it.

[vstinner]

Please fix tox -e pep8 errors:

 hachoir/parser/misc/ttf.py:340:5: F841 local variable 'start' is assigned to but never used
hachoir/parser/misc/ttf.py:404:18: F821 undefined name 'er'
hachoir/parser/misc/ttf.py:405:12: F821 undefined name 'last'
hachoir/parser/misc/ttf.py:405:21: F821 undefined name 'last'
hachoir/parser/misc/ttf.py:407:9: F841 local variable 'last' is assigned to but never used
hachoir/parser/misc/ttf.py:637:39: F821 undefined name 'parseFeatureVariationsTable'
ERROR: InvocationError for command /usr/bin/sh tools/flake8.sh (exited with code 1)

[simoncozens]

I think this is done now, with a few more fixes in there and another table for good measure.

[vstinner]

Merged, thanks.