Added "Save Image"

Image passages can be saved as their original files using the "Save
Image" menu option in the image window's menu.
Merge pull request #2 from tmedwards/master

Fixes for <<print>> and <<choice>>
Added "Special Passages" Submenu

* Added a submenu to Story, "Special Passages", which lists the various
info passages (StoryTitle, StoryAuthor etc.) and creates one with
default text if it is chosen.
* Fixed bugs involving the obfuscation setting.
* Creating a new Twine project now creates empty StoryTitle and
StoryAuthor passages in addition to the Start passage.
Merge remote-tracking branch 'upstream/master'