Change int2float, float2int of cortex_m for 64bits

Change pack/unpack from L to I in int2float and float2int for cortex_m.
L is 8 bytes long on 64 bits amd64 python (2.7.5+ GCC 4.8.1 Linux).
Merge pull request #29 from ynsta/master

Change int2float, float2int of cortex_m for 64bits