Add set_long and get_long functions

They work on platform-dependent long ints
fix compiler warning
fix macros
fix compiler warning when compiling with "-mx32" option
add tests for mp_set_long() and mp_get_long()
update makefiles etc.
Merge branch 'moritz/get-set-long' into develop

This closes #26
less noise plz
replace mp_set_long() implementation by macro
add mp_get_long_long() and mp_set_long_long()
add tests
udpate makefiles etc.
fix compilation with MP_8BIT or MP_16BIT