Patch version 6:

 - timestamp format is now a type instead of a string, e.g. time.time(int)
 - add int and datetime.timedelta formats, remove timespec format
 - complete the documentation
 - fix integer overflows, convert correctly time_t to PyLong

There are now 5 timestamp formats:

 - int
 - float
 - decimal.Decimal
 - datetime.datetime
 - datetime.timedelta

I consider the patch as ready to be commited, or at least ready for a
review ;-) There is no more FIXME or known limitation. Well, now the
most important part is to decide the API and the list of timestamp
formats.

The patch should be tested on Linux, FreeBSD and Windows, 32 and 64
bits to check assertions on type sizes:

assert(sizeof(clock_t) <= sizeof(size_t));
assert(sizeof(LONGLONG) <= sizeof(size_t));
assert(sizeof(time_t) <= sizeof(PY_LONG_LONG));