Version 2.4.6: printing and empty/None formatting closer to Python's own

The string and empty format type formatting of numbers with uncertainties are now similar to what Python 3.4 does (https://docs.python.org/3.4/library/string.html#format-specification-mini-language).

The previous behavior was based on the Python 2.7 documentation, which is probably incorrect (http://stackoverflow.com/questions/16525924/precise-definition-of-float-string-formatting).