Markdown: Content Item Implementation

Implemented complete markdown editing and display for exhibit content
item descriptions, including preview, help and hints.

Implemented markdown editing and display for tour stop descriptions,
with preview, help and hints to follow.
Merge branch 'NeilCresswell-markdown'
Markdown: Tour Stop Implementation

Added Markdown help and hints to tour stop editing panel. Also changed
Markdown editor inputs (not rendering) to be monospaced so can support
features such as table layout.
Merge branch 'markdown' of git://github.com/NeilCresswell/ChronoZoom into NeilCresswell-markdown
Merge branch 'NeilCresswell-markdown'