Change to standard save icon in Notebook toolbar

Closes gh-3545
Merge pull request #3548 from takluyver/save-icon

Change to standard save icon in Notebook toolbar

What used to be a hard drive image is now the more traditional floppy image
Added writters base, started modifications of extractfigure
Removed trivial text data types (doesn't include SVG), templates should pull directly from nb struct. and only extract data that must be reference via filename.
Added multi-notebook support to figure extraction methods.
Added file writer.
Fix, .items() added to resource dict to allow iteration.
Fixed bad ref., incorrect base name.
Better comment
Added notebook name parameter and description
ExtractFigureTransformer is now automatically constructed again...
The notebook_name is set through a variable now so the ExtractFigureTransformer doesn't
have to be constructed for every notebook being exported.
Basic support for writers, still no YAML
Fixed comment typo
Added check to make sure notebook_name is not null
Notebook name no longer set through constructor
Whoops, forgot to commit this line with the last commit
Fixed typo, wrong variable name.
Fixed import typo.  Remove ".py" doh!
Added Stdout writer.
Made referenced files list configurable.