rename `%%file` to `%%fwrite`

keeps `%%file` as alias

Alternative to #3313, no behavior is changed.
make that writefile
Moving handlers to subdir
Adding new files.
Splitting handlers into different files.

I have also created a top-level utils.py for notebook related stuff
Adding __init__.py
Fixing url join problems.
Removing debug message.
define `__file__` in config files
More work on the handlers
Reorganizaing the server side of the notebook.
Updating import statements after moving notebook files around.
Renaming api handlers and moving FileFindHandler into base handlr.
Renaming templates to match other names.
Moving base ZMQ handlers to base/zmqhandlers.py.
Removing dup import.
clarify FIXME comment for magics aliases
Refactor notebookapp __init__ method.
Merge pull request #3317 from minrk/fwrite

Rename `%%file` to `%%writefile`, leaving `%%file` as an alias for backwards compatibility.
Updating tests for refactor.