Allow multiple unicode values for glyphs

[khaledhosny]

Fixes #85

[coveralls]

Coverage decreased (-0.04%) to 88.609% when pulling 4d05265 on khaledhosny:unicodes into ace7445 on LettError:master.

[khaledhosny]

Not sure if this breaks public API, if so we probably need to keep unicodeValue and provide another unicodes keyword argument similar to defcon. Also I think the old code was trying to detect different unicode values accross masters, which is probably not detected now.

[anthrotype]

the designSpaceDocument's specification should probably be modified as well to allow a list of unicode values for each glyph element, rather than a single one:

https://github.com/LettError/designSpaceDocument#attributes-7

[anthrotype]

@khaledhosny @LettError so, does this change the attributes of glyph element (from unicode to unicodes) in the generated designspace? Shouldn't it be reflected in the spec?

btw, the designSpaceDocument still uses the old unicodeValue:
https://github.com/LettError/designSpaceDocument/blob/master/Lib/designSpaceDocument/__init__.py#L897-L900

[LettError]

Thanks @anthrotype, guess I'll need to fix that.

[LettError]

I think I made all the right changes to support multiple unicode values in the glyph element in designSpaceDocument. Tests added.
https://github.com/LettError/designSpaceDocument/compare/unicodes
LettError/designSpaceDocument#21