Before this change the Part structure had an int for the Offset and
uploading large files would produce this error
json: cannot unmarshal number 2147483648 into Go struct field Part.offset of type int
Changing the field to an int64 fixes the problem.
Very large directories can have their sizes returned as floating point
numbers, eg `1.0034576985781e+14` from the box API.
Before this change this would fail to parse as an int64.
This change parses the size as a float64 instead which will be
perfectly accurate for sizes up to 2**56 which is about 9 PB.
It is unknown whether box themselves use a float64 as an intermediate
representation in the API or not - it seems likely.
Fixes#2261
