Automatically update the Web Starter Kit latest release download URL

Grunt 'build' will trigger the 'wsk-version' task, which in turn uses
['latest-release'](https://www.npmjs.org/package/latest-release) to
determine the name, tag and zip-archive download URL of the latest
release of google/web-starter-kit on Github.

The results of this are written to src/site/_config-wsk-version.yml.

Jekyll.destination.config has been expanded to pick this config yml up
load in the variables when building the site.

src/site/_en/starter-kit/index.html uses the {{site.wsk-zip-url}}
variable to point it's download button the to latest version.

See discussion in:
- google/web-starter-kit#414
- google/WebFundamentals#547
Bumps Web Starter Kit to v0.5.1

Commits the generated output of `grunt wsk-version`.
corrected some translation of constructing-the-object-model.markdown
Add material for Chrome 39 beta.
Merge pull request #915 from jpmedley/M39

Add material for Chrome 39 beta.
Add sample layout link

Fix #910
Merge branch 'master' of https://github.com/google/WebFundamentals
add images-in-css.markdown to _zh-cn
translate images-in-css.markdown into Chinese
Fix issue #918

As reported by @nebloglazov, the article should include the CSS
declarations of the classes being toggled.
Merge pull request #919 from jacquerie/issue-918

Fix issue #918
Merge pull request #916 from chenxsan/media

translate images-in-css.markdown into Chinese
Fix assignment in condition

Corrects the use of assignment operator ('=') rather than boolean ('==') in conditional expression.
fix typos
Merge pull request #921 from chenxsan/fix-typos

fix typos
add use-icons.markdown to _zh-cn
Translate use-icons.markdown into Chinese
Merge pull request #922 from chenxsan/use-icons

Translate use-icons.markdown into Chinese
meta data change only - added priority & cleaned up yaml
meta data change only - added priority & cleaned up yaml