Fix #14: Make Overview Editor default editor for .salt files

- Set default to false on AtomicSaltEditor, and to true on
OverviewEditor
- Set extension to "salt" for OverviewEditor
- fix #14

Bug: http://github.com/infraling/atomic/issues/issue/14
Merge branch 'overvieweditor' of
ssh://git@github.com/infraling/atomic.git into overvieweditor