[casting] read/write custom parameters as generic numbers (int or float)
[casting] read/write custom parameters as generic numbers (int or float)

As Georg said in
https://github.com/googlei18n/glyphsLib/pull/110#issuecomment-278892168
for custom parameters, Glyhps.app does not enforce rounding and nor it
distinguishes between float and int.

The rounding, if any, should be handled in ufo2ft when generating the OTF.

ufoLib already validates the UFO attributes types, but we shall make sure
that glyphsLib does not pass float when ufoLib expect ints, or viceversa.

Fixes https://github.com/googlei18n/glyphsLib/issues/114

googlefonts/glyphsLib,googlei18n/glyphsLib