Added a FileBlock.to_action for create.

This is a simple version of FileBlock.to_action that does minimal
safety checking, and handles file creation using a ShellAction. A more
specialized Action might be more appropriate in the future.
Added ActionExceptions when ShellActions fail.

This includes some tweaking to ActionException and the action testsuite.
The testsuite now uses a MockProcess object to supply dummy returncode
and wait() attributes.
Merge branch 'feature/file-create-action' into dev