Don't use form in ServerAdmin

Instead use raw ModelAdmin. It helps if there is need to
manually change is_measured flag.
Measure also cached memory in default memory sensors.

Previous version was giving extremely high "used memory"
measures.
use reverse resolver to determine skwissh LOGIN_URL

Still patching django.conf.settings but now it is possible to
place skwissh anywhere - not only under r'^skwissh/'.
Merge pull request #15 from swistakm/master

Thanks for your help...