added basic page action assets

Markdown support

Fix #91

2. Re-added an accidentally removed line

- Open links in a new window/tab.
- Fixed some Ubuntu instructions.

Thanks @csillag

Changes made:

Until so far, the "details" and "summary" Handlebars templates referenced
the "text" field of the annotation object, and they automatically
HTML-escaped the content found there.

Since now we are placing HTML unsafe content there (the links),
this had to be changed. So now the templates are using triple-stash
(which means no automatic escaping), and reference the "rendered_text"
field of the annotation object.

This rendered_text is filled by manually escaping the text of the
annotation body, plus making the URLs clickable.

Just toying with the concept of separating h as deployed out of the
box from deploying pieces of it as a pyramid library. Separates the
application from the library by keeping application-related things
in the main module. <shrug>