use magit-define-command for magit-svn-* commands

so that we can use switches and pass them through magit-custom-options
add 'switches key in empty key-mode groups

This patch modifies `magit-key-mode-add-group' so that a new empty group
includes a 'switches list which then allow us to use
`magit-key-mode-insert-switch' in order to add switches to new
groups (or existing groups with no default 'switches) during runtime.
add --dry-run switch (-n) to magit-svn