Make GlyphsBackend writable

[justvanrossum]

This will be done in multiple steps.

• Make GlyphsBackend writable for glyph data #75
• Support background layers #88
• Support deleting glyphs #90
• Font info
• Variation axes
• Font sources info
• Make kerning writable #98
• OT features Read and write OpenType features #95
• Units Per Em
• Custom data

(All putXxxx() methods that now raise NotImplementedError)