Add __repr__ for easier reading of printed IPs.

This adds [eval-like `__repr__`](http://stackoverflow.com/a/2626364/543431) strings to `IP()` and `Network()`, making it a lot easier to compare IP's when printed.

```
<ipcalc.IP object at 0x...>
```
vs
```
IP('127.0.0.1', mask=32, version=4)
```