Additional test: sign, padding with zeros, minimum width, percent (and NaN uncertainty).
Complete (but not tested): width handling for the shorthand notation.
Comments added about the .6g format, which does not give 6 significant digits, but only a *maximum* of 6 digits (same for other numbers).
Added tests with the comma format specification option.
(Not tested) Better handling of the 'n' format.
Added tests for the "n" (locale) format.
More consistent variable name.
Removed the incorrect term "mantissa".
Fixed repr() for UFloats.
Added test for the C formatting option, which only works with Unicode strings.