New way to detect incoming worker connections

We use the request.remote_addr value to determine the request.
Apparently it's very hard to get the IP address of a machine
consistently on OSX and Linux. Probably there will be problems when we
work with non LAN IPs but for the moments it seems to work.
Worker system detection

WARNING: this commit requires the addition of a 'system' column in the
workers table.
When a worker is started we detect the operating system and the
version. This is useful for sending the right commands to workers later
via the server.
worker page : Fix check all checkbox
job page: Fix check all checkbox
adding check if no shot is selected

right now I just do a javascript alert, it would be good to implement a
messaging system later ( a bit like flask.flash(), but on the html side.
quickfix : enable disable button was broken

after adding the system column we forgot to offset the fnUpdate to
column 5