trigger `Kernel.status_started` after websockets open

This leads to a more logical state for event watchers to expect.
Fix citation2latex for Python 3
Fix pylabtools test for Python 3
remove special handling of speaker notes
clarify `reveal-prefix` alias name
update nbconvert docs about reveal.js
add proxying tornado server in Serve post-processor
use local reveal.js url prefix by default

CDN redirect is handled by the Serve post-processor if necessary
add missing data_javascript

Note that javascript is still excluded from html output by default.

candidate for backport to 1.1
minor adjustments to markdown / rst templates

- remote prompts from output
- add missing HTML block
- remove double-wrapping of latex
- use plain-markdown indentation for code blocks (could use GFM '` ` `python')

closes #4024

candidate for backport to 1.1
fix markdown images

use split-files for embedded images

add path2url
fix jpg/jpeg type in sphinx
fix skeleton template makefile
run skeleton template makefile
Merge pull request #4168 from takluyver/py3-failing-tests

Fixes the failing tests revealed by #4165.
ESC should be handled by CM if tooltip is not on
Have remove_and_cancel_tooltip() return a boolean
Merge pull request #4178 from minrk/data_javascript

add missing data_javascript
Merge pull request #4174 from minrk/markup-templates

various issues in markdown and rst templates

- remove prompts from default output
- add missing HTML block
- remove double-wrapping of latex
- use plain-markdown indentation for code blocks (could use GFM ``` ` ` `python```)
- images didn't work in either one at all
- markdown extracts output

closes #4024
converted multi-context with-statements to nested (2.6 compatibility)