Fix processing of dataencoding for 3-byte integers

There was a unit test for 3-byte integers, where the number 99
(0000_0110_0011)b was successfully decoded. Changing the number 99
with for example the number 236 (0000_1110_1100)b would make the unit
test fail. The reason being that the 3-byte integer is read in two
goes (first a short, then a byte). Both of these are read as signed,
whereas we want to be working with the unsigned version.
Add support for 64-bit integers

The code already supported up to 32-bit integers. This commit
extends this to 64 bits. There is no change in the XTCE spreadsheet
but the value may need to be retrieved using Value.getUint64Value()
if the size of the dataencoding exceeds 32 bits.
Merge pull request #13 from fqqb/integer-processing

Integer processing