Update ga2 snippet

Save a few bytes using HTML5 Boilerplate version [1].

[1] https://github.com/h5bp/html5-boilerplate
Update html snippet

Based on HTML5 Boilerplate [1]. Also add `lang` property in HTML tag.

[1] https://github.com/h5bp/html5-boilerplate
Follow GitHub HTML guidelines about attributes

"Even though quotes around attributes is optional, always put
quotes around attributes for readability." [1]

[1] https://github.com/styleguide/templates
Add placeholder and update default jQuery version
Change tabTrigger for HTML snippet
Update README.md and install.txt
Remove `meta` snippet and add `head` snippet instead

The `head` snippet add a `HEAD` section based on HTML5 Boilerplate [1].

Also update README.md and install.txt.

[1] https://github.com/h5bp/html5-boilerplate
Update "nav" snippet

Remove comments.
Remove `nav2` snippet
Follow GitHub HTML guidelines about attributes

"Even though quotes around attributes is optional, always put
quotes around attributes for readability." [1]

[1] https://github.com/styleguide/templates
Update README.md

Remove `meta` since it does not exist anymore.
Add new `doctype` snippet