Tweak %load to get raw gist #2149 

These changes tweak the load magic to turn gist url into raw-gist url since it doesn't make sens to load an html gist page. This change is in response to Issue #2149 (https://github.com/ipython/ipython/issues/2149). 

![before](https://cloud.githubusercontent.com/assets/4523537/2755977/e5e1531c-c96f-11e3-80f0-31e65b05b3ac.png)
![after](https://cloud.githubusercontent.com/assets/4523537/2755976/e5dc8c1a-c96f-11e3-85c4-e74fa02491c7.png)