Big rewrite of frontend code

credits to @travigd

revert

v0.7.0

special case on asset keys + still throw if file not found

v0.4.2

special case on asset keys + still throw if file not found

v0.4.1

wait for both connection and onimport before accepting messages

add generic http server

...for simpler implementation of show + display for 3rd party provider

v0.3.4

compat for latest JupyterLab. closes #171

robust comm setup on IJulia

v0.3.3

Create Comm from the JS side rather than Julia side.

Fix comm setup on IJulia

v0.2.8

Create Comm from the JS side rather than Julia side.

Fix for JuliaBox on Julia 0.6

v0.2.7

also classify //foo as non-relative

v0.3.2

allow custom host (#186)

* allow custom host

* make it more backward compatible