Add simple function to return system stats.

Add a common function that all Go projects can use to return useful debugging information. We can add more stats as we see fit.
Good point. What you prefer to just have it documented or store the units type in the struct itself?

We could have something like:
```
{
  "NumGoRoutines": {
    "Value": 7,
    "Units": "number"
  },
  "UserTime": {
    "Value": 0.004,
    "Uints": "cpusecs"
  },
  "SystemTime": {
    "Value": 0,
    "Uints": "cpusecs"
  },
  "MemoryAlloc": {
    "Value": 649464,
    "Uints": "bytes"
  },
  "MemoryFromSystem": {
    "Value": 4331752,
    "Uints": "bytes"
  }
  ...
}
```

This is really the same information as https://github.com/fastly/go-utils/blob/master/ganglia/ganglia.go#L282 - might be able to merge the two as well.