Analysis incorrect for negative numbers

This patch fixes the bug and cleans up the code a bit.

1) Counts negative number in statistics.
2) Renames $nonzero and $not_empty to $non_zero and $non_empty for consistency.
3) Uses (float)$value to handle empty strings. No special handling is needed: http://www.php.net/manual/en/language.types.string.php#language.types.st...

The only slight remaining weirdness that I see is that the option to consider empty/zero is used to switch from a straight std dev to a sample std dev by dividing by N-1. I'm not sure that this is justified, especially in the case where there are no non-empty value. See http://en.wikipedia.org/wiki/Standard_deviation. If it's thought that this isn't the best solution, another issue should be created. I don't purport to be a statistics expert.

Committed #1 to 7.x-4.x and backported to 7.x-3.x and 6.x