Added sorting support by clicking on table cell header

As discussed in #184 I have implemented some simple sorting which can be merged in.

There is one thing I'm still not happy about (because I did not understand how to make it work):
I would like to have  default compare value method, something like: 
```return col.getCellContent(b) - col.getCellContent(a);```
but for whatever reason the content (a,b) are not ember objects and then they do not define the ```.get()``` method so the default implementation of  ```getCellContent``` fails.


should probably observe `content` as well?