SMART — change time format in order to make the offset obsolete

Even the smallest 8-bit microcontrollers should be capable of dealing with signed numbers in the 2-complement representation. Thus the following changes are proposed:
In Section 2.3.8 "... plus an offset of 2^38 seconds. The offset is introduced to be able to express instants before January 6, 1980 as positive values." should be replaced by "Instants before January 6, 1980 are represented as negative values."
The IDL in Section 3.4 should be changed in the following way:
"typedef unsigned long long TimeInstant" should be replaced by "typedef long long TimeInstant" and "typedef unsigned long long TimeDuration" should be changed to "typedef unsigned long long TimeDuration"; in addition the description in Section 3.5 must be changed

Accept changes as proposed.