Upconvert UFO v1 and v2 kerning data

[madig]

Part of #31.

• Spec: http://unifiedfontobject.org/versions/ufo3/kerning.plist/#converting-to-ufo-3-formatted-kerning
• Reference implementation: https://github.com/fonttools/fonttools/blob/master/Lib/fontTools/ufoLib/converters.py

TODO:

• Make it work, then make it not copy everything
• Run kerning group validation after conversion
• Implement all reference implementation tests (Tests/ufoLib/UFOConversion_test.py!)
• Move into own utility/conversion module?
• What if the target kerning group exists already? Making the names unique won't save you from then potentially having glyphs in more than one kerning group. The reference implementation already fails here. Error out
• What if either groups.plist or kerning.plist exist but not both? The algorithm needs groups.plist at minimum, kerning.plist can be empty. Pass in empty container if kerning is None.

[madig]

ufoLib fails itself when you up/downconvert UFOs from v2 -> v3 -> v2 and load the last UFO again. Since it leaves all groups intact, upconversion will hit duplicate group names and make new ones with the same glyph list. Validation will subsequently fail.

In [3]: u = defcon.Font("sourcesanspro_v2.ufo")
In [4]: u.save("/tmp/sourcesanspro_v3.ufo", formatVersion=3)
In [5]: u2 = defcon.Font("sourcesanspro_v3.ufo")
In [6]: u2.save("/tmp/sourcesanspro_v2_2.ufo", formatVersion=2)
In [7]: u3 = defcon.Font("/tmp/sourcesanspro_v2_2.ufo")
[...]
UFOLibError: The glyph "nu" occurs in too many kerning groups.

[cmyr]

Okay a couple little nitty things but overall this looks good!

[cmyr]

okay a couple little things you can try if you like, otherwise I'm happy!