Display async cores too

By default display only the workers, as usual, but with a 'a' key
press async core display can be enabled. Pressing 'a' again will switch
display mode, and another 'a' will return back to the default display.

fixes #12

Tested only under Python 2.7.

Pressing 'a' will switch to such a view:
![merged_view](https://f.cloud.github.com/assets/765347/2256405/9975f0f8-9e06-11e3-8c43-cab04cce2d46.png)
Another 'a' will switch to this:
![separate_view](https://f.cloud.github.com/assets/765347/2256407/9db54ff6-9e06-11e3-9b7f-acadf75b4d97.png)
And, another one, back to default:
![default_view](https://f.cloud.github.com/assets/765347/2256410/a128ac0a-9e06-11e3-9e14-1cd7aba93372.png)

'1' is the worker-id, ':97', ':98', ':99' are the core ids. Cores that haven't handle any requests ('0'..'96' in this case) are not displayed.