Implemented run python script using saltpig

- Added simple dart API for for wamfs
- Added action to run the selected python script in the files view.
It will run the python script through saltpig. The output will be on the console.
Also it's only available in developer mode.

review: @rginda @devoncarew @gaurave

The current version of saltpig needs to be installed and configured for that to be working.
The 6 methods here differ only in `callMethod` arguments: they repeat 90% of the boilerplate. Could you factor out a lower level worker method, `_callWamFsMethod(String name, List arguments)`?