Set html lang attribute.
Update layout.
Update index page.
Add installation page.
Set Cask's logo to a fixed position.
Update usage section.
Add API section.
Remove Cask image.
Add Cask-file screenshot.
Add DSL section.
Remove old index page.
Add examples section.
Add tools section.
Add troubleshoot section.
Add license section.
Add development section.
Add why cask section.
Merge branch 'v0.6'