Mention Tizen _Core_ APIs, not native APIs, in Tizen IVI ext tutorial

As per comment on https://crosswalk-project.org/jira/browse/XWALK-1187
Mention alternative ways to run Tizen IVI
Add note about gcc and rootstrap to Tizen IVI extensions tutorial
Incorporate wiki content about the C interfaces

Move some of the content from
https://github.com/crosswalk-project/crosswalk-website/wiki/Writing-a-Crosswalk-Extension
to the official docs.
Merge pull request #77 from townxelliot/tizen-ext-tutorial-fixes-1187

[do not merge] Tizen ext tutorial fixes