[modules/memory] Use MemAvailable, if present
[modules/memory] Use MemAvailable, if present

If the kernel supports it, MemAvailable contains an estimation
of the memory available for usage.

Use this to calculate the amount of free memory (as this seems to
closely match the output of gnome-system-monitor).

fixes #253

tobi-wan-kenobi/bumblebee-status,tobi-wan-kenobi/bumblebee-status