Use multiprocessing to parallelize switch startup

To get the ball rolling for improving link creation and switch startup times in Mininet, this simple patches parallelizes the switch creation by using the `multiprocessing` module. I have tested this patch with:

```
# mn --version
2.1.0+
```
```
# ovs-ofctl --version
ovs-ofctl (Open vSwitch) 1.4.6
Compiled Jan 10 2014 01:45:55
OpenFlow versions 0x1:0x1
````
and using the `POX` controller. As a comparison: it takes 154 seconds to start 100 `OVS` switches without this patch but it takes 65 seconds with the patch applied (on a really powerful machine). CPU usage with the patch is ~98-99% on all cores as the switches are being created.

I am sure we can do better, but I am submitting this request so that we can discuss it further. Also, for whatever reason, I **cannot** get this to work with `OVS 2.1.0`:

```
# ovs-ofctl --version
ovs-ofctl (Open vSwitch) 2.1.0
Compiled Apr  1 2014 02:06:32
OpenFlow versions 0x1:0x4
```
Some switches are created, but there are socket errors for others. Suggestions on fixing this for `OVS 2.x` are also welcome.