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.
A more relevant virtualenv link for installation
wording - fix usage of the "its" adjective
Merge pull request #31 from webcracy/its-hunt

Wording -- fix usage of the "its" adjective
Merge pull request #30 from tiflopin/master

A more relevant virtualenv link for installation
Refactor custom choice parameters to improve global speed of commands.

Only retrieve parameters choices list when needed, as displaying help
or running the command. This skip retrieving unused parameters for a
command and thus greatly reduce number of API calls.