v1.2

Various tweaks and fixes all over

• Update README.md with new tweaks
• Add title to default template
• Update javascript includes with newest site-behaviors
• Rename twitter_username to site_twitter_username
• Rename site breakpoints to be more descriptive
• Add background-image mixin
• Move statamic-js into site-behaviors and add theme js
• Add favicon tag to layout
• Update global typo in theme yaml