Added the ability to run external files in the context of qtile

Example:
In an external editor, I write code for qtile and I want to test it without restarting qtile.
For example in the file /tmp/test.py I write:
```python
def main (qtile):
     print qtile.currentLayout
     qtile.cmd_nextlayout ()
     print qtile.currentLayout
```
then run: qsh -r /tmp/test.py
and get to stdout:
```shell
<libqtile.layout.xmonad.MonadTall object at 0x7fc06a964c50>
<libqtile.layout.max.Max object at 0x7fc06a964bd0>
```
I don't think we want to force this to be python2; if it doesn't work in python3 we should fix that instead (although it doesn't have to be in this patch).