Throttling and measuring for writers/readers

The idea is to be able to throttle the rate at which an io.Writer/io.Reader transfers data, and also monitor that rate.  The output of the Go example looks like:

```
=== RUN: Example_ThrottledReader
2014/09/30 23:55:23 reading at 195KiB/s
2014/09/30 23:55:23 reading at 184KiB/s
2014/09/30 23:55:23 reading at 99KiB/s
2014/09/30 23:55:23 reading at 99KiB/s
2014/09/30 23:55:23 reading at 99KiB/s
2014/09/30 23:55:23 reading at 99KiB/s
2014/09/30 23:55:23 reading at 99KiB/s
2014/09/30 23:55:23 reading at 99KiB/s
2014/09/30 23:55:23 reading at 0B/s
--- PASS: Example_ThrottledReader (0.10s)
=== RUN: Example_ThrottledWriter
2014/09/30 23:55:23 writing at 199KiB/s
2014/09/30 23:55:23 writing at 99KiB/s
2014/09/30 23:55:23 writing at 99KiB/s
2014/09/30 23:55:23 writing at 99KiB/s
2014/09/30 23:55:23 writing at 99KiB/s
2014/09/30 23:55:23 writing at 99KiB/s
2014/09/30 23:55:23 writing at 99KiB/s
2014/09/30 23:55:23 writing at 99KiB/s
--- PASS: Example_ThrottledWriter (0.08s)
PASS
ok  	github.com/aybabtme/iocontrol	0.192s
```