Round up floating point offsets

Some browsers (IE, maybe Firefox) return floating point numbers from
$.offset() calls. The affix plug-in compares two offsets to determine
when to apply .affix-bottom. Under the right (wrong) conditions, that
comparison can fail due to the imprecision of floating point math.

We attempt to head that off by rounding up one of the two offsets (the
one that we can control).

Fixes #5.