Add tutorial for embedding API
Link to new tutorial from embedding API tutorial page
Remove section about HexGL until permission received

Plan to use HexGL as an example of a full application loaded
using the embedding API. However, I haven't got permission yet
to do this, so remove the relevant sections until I get that
permission. This will enable me to push this branch to live
in the meantime.
Minor edits for Crosswalk embedding tutorial
Merge pull request #117 from crosswalk-project/embedding-api-1740

Add Embedding API tutorial
Make replace_version_string global

This is so I can use it in the manifest page to get the current
major version number.
Add manifest overview page
Add sub-pages for field descriptions