add uptime and loadavg files

[tschuy]

Closes #60.

As far as I can tell, we don't currently collect the uptime. It's not a particularly expensive syscall to just grab /proc/uptime: https://github.com/torvalds/linux/blob/5924bbecd0267d87c24110cbe2041b5075173a25/fs/proc/uptime.c

tschuy@tschuy:(mayday)(uptime) → cat test.c
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    int i;

    FILE * fp;
    for (i = 0; i < 1000000; i++) {
        fp=fopen("/proc/uptime", "r");
    fclose(fp);
    }
    return 0;
}
tschuy@tschuy:(mayday)(uptime) → time ./getuptime 

real    0m3.516s
user    0m0.257s
sys 0m3.250s

3.5 microseconds per read for uptime, and very similar timing for loadavg.