Added writers and supporting code.
Merging done by hand
Added writer classes
Missing sys import
Fixed unicode data not written to files properly.
Catch export specific name error only
Added, can be removed later in a different PR.
base64 encode PDF before writing back to the nb structure
Make copy of notebook node object to make sure the source doesn't get modified
Fixes "bugs with mutable defaults args", thanks @Carreau
Indent fix
Explicitly set enabled=True
Changes after in person review with @ellisonbg including TODO tags
Renamed svg2pdf converting transformer
Removed activatable, moved enabled to base
Prefer new style of config passing, parent=self (replaces config=self.config)
Transformers in traitlet lists now, new _init_ methods,
default metadata dict
Mac OSX support for svg converter
Missing imports
To and from formats configurable