remove notebook read-only view

it is largely broken, and had problematic security issues.
Add cwd to sys.path
This makes transformers and filters in the cwd load
from dotted object names.
Better name `init_syspath`
add signature_scheme to Session and connection files

also change default hash to sha256 instead of md5, since md5 is bad.
(this is the main motivation)
propagate signature_scheme config to parallel connection files
document the wire protocol
remove unimplemented message types from the messaging doc
Insert cwd to top of list
Merge pull request #3743 from minrk/noro

remove notebook read-only view

it is largely broken, and had problematic security issues.
remove bad `if logged_in` preventing new-notebook without login
Merge pull request #3779 from minrk/new

remove bad `if logged_in` preventing new-notebook without login

Min's mistake :-)
comply with the message spec in object_info requests

missing detail_level key
add missing block key in complete_request
fixes to completion and connect messages
set some topics on IOPub messages

these are informally defined and have no effect,
but we should be a bit better with them.
add some detail to the wire protocol
add Importing Notebooks example
add missing comma
Raise a named exception when pandoc is missing

In the normal case this will case the interpriter to exit just like
sys.exit(...), but it will make it possible for callers to explicitly
catch the exception too.

Also, removed parens from the pandoc URL so it's easier to
triple-click-copy-and-paste.
Catch ConversionExceptions in main nbconvert loop