max_memory param for update, suggestion method

provide a suggestion method to estimate the best max_memory
value, and add max_memory into vm_update
add max_memory handling in vm_update

Do not expose internal max_memory constraints, as this will
probably change someday. Default to forbid update when online
resize is not possible, add --reboot to acknowledge the offline resize.
add documentation about vm update --reboot
fix undefined max_memory needed for the update call
ask confirmation for offline updates rather than exiting
use info method rather than an api call
Merge pull request #28 from kalou/maxmem

Update gandi vm update command to handle case when a memory update triggered a vm_max_memory update.