Introductory text for Embedding JS

Adds new directories and files:

- /syntax_and_idioms
- /syntax_and_idioms/README.md
- /syntax_and_idioms/embedding_js
- /syntax_and_idioms/embedding_js/README.md
Use smaller HTML example in embedding_js; add references
First three exercises for embedding JS
Add exercises on using JS to write text inline
Exercises for loading external JS files (incl. jQuery from CDN)
Merge pull request #6 from codeunion/embedding-js-in-webpage

Embedding JS in webpage