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>
```
This line is not important because qsh runs from python2 script:
```shell
# ~$ where qsh       
/usr/bin/qsh

# ~$ cat /usr/bin/qsh
#!/usr/bin/python2
# EASY-INSTALL-SCRIPT: 'qtile==0.8.0','qsh'
__requires__ = 'qtile==0.8.0'
__import__('pkg_resources').run_script('qtile==0.8.0', 'qsh')
```