Update packaging etc

[ceball]

• fixed dependencies
• dev tag should result in conda package on pyviz/label/dev (there's one already I did as a test from the branch)
• website should only update for changes to master (it was updating for any build before)
• added placeholder for unit tests, and switched to pytest
• py2 fixes
• misc cleanup

Note: a failing notebook (layouts) is currently being skipped.

To do:

• need a starting tag on master for version to work?
• skip failing layouts notebook
• test a dev tag package build
• fill in ANACONDA_TOKEN=(pyviz token) at https://travis-ci.org/pyviz/panel/settings
• py2 fixes
• after merging this PR, check that website build happens for master

Future PR(s):

• pip packages (once param is released)
• test examples with conda package (what dependencies should conda package have?)
• switch to pyctdev's standard (internal) conda recipe
• figure out the extra stuff in the website build
• add windows/appveyor

[philippjfr]

Thanks for cleaning up my mess 😄