also fix du for file size (use 'wc -c')
fix logic on file staging
add purge_on_start
more documentation work... ugh.
working mandelbrot tutorial example
Support for protected ssh keys, better default context detection, fix shell bootstrap

This commit fixes #66, and resolves a coupld of related issues with ssh context
handling.  Tested with various ssh key types, but additional tests would be
appreciated.
Merge branch 'devel' of github.com:saga-project/saga-python into devel