Implement toggle of process list display

Parsing and displaying the process list takes a sometimes significant
amount of resources.  Since there is already a mechanism in place to run
glances with the process list disabled, it was easy to add a toggle to
the __catchKey function.
Adapt to psutil 2.0.0 API.

API changed.
https://psutil.googlecode.com/hg/HISTORY

Tested with 0.5.1, 1.2.1, 2.0.0
Fix python trying to use default getattr using psutil 0.5.1.

Don't know why adding "None" as second default helps.
Fix error types.
Fixed psutil 0.5.x support on Windows
Merge pull request #337 from regdoug/windows-psutil-0.5

Fixed psutil 0.5.x support on Windows
Merge pull request #336 from herr-biber/adapt-to-psutil2.0.0-api

Adapt to psutil 2.0.0 API.
PsUtil 2.0 is back for the branch 1.x
Merge pull request #335 from weaselkeeper/master

Implement toggle of process list display
Add z key in the documentation