Add some fudgeFactor to offset comparison arithmetic

Some browsers (IE, maybe Firefox) return floating-point offsets, which are not precise
enough when we want a comparison between two offsets to exactly equal zero. The affix
plug-in makes such a comparison to determine when to apply .affix-bottom. Under the
right (wrong) conditions, the difference between two offsets is just slightly less than
zero, causing affix to apply .affix instead of .affix-bottom.

Adding some fudge factor to the offset seems to prevent that.

Also, use .offset() instead of .position, for consistency.

Fixes #5.