Problem: str2nr() and str2float() do not always work with negative values.
Solution: Be more flexible about handling signs. (LemonBoy, closes#1332)
Add more tests.
Problem: On MS-Windows some float functions return a different value when
passed unusual values. strtod() doesn't work for "inf" and "nan".
Solution: Accept both results. Fix str2float() for MS-Windows. Also
reorder assert function arguments.
