use arc4random() instead of rand() on *BSD
Merge remote-tracking branch 'carbin/arc' into develop

This closes #28
add testme.sh
add documentation of mp_expt_d_ex()
trim trailing spaces
update changes.txt
update documentation

add mp_set_long, mp_get_long, mp_set_long_long, mp_get_long_long
update changes
clarify documentation of mp_div_2d() and mp_mul_2d()