Add index.js, front-page buttons.

Add:

	templates/index.html, static/js/index.js, static/css/_index.sass
		-add radio-buttons for Custom/Premade tour and Bike/Foot transportation.
		-add jquery animation for buttons to "cascade" downwards as they
		bottom-most row is checked.

	static/css/_social_media.sass
		-change distance of socialMedia div from the top of the screen to a
		static pixel value, instead of a relative percent measure, for precise
		alignment with the .main-content.content div regardless of screen
		height.

Ref:
	templates/index.html, static/js/index.js
		-move inline javascript from index.html to index.js, for clean
		modularity.
Add imgs to front-page labels, hover/click effect.

Add:
	templates/index.html
		-add img tags (with placeholder images ) to cascading button labels.

	static/css/_index.sass
		-add style for button label images.

	static/css/_mixins.sass
		-add mixin for opacity (has an additional line for <=IE8 support).
Darken opacity of radio label images.

Mod:
	static/css/_index.sass
		-darken opacity of radio label images.
Merge branch 'front_page_ui'

Conflicts:
	static/css/_variables.sass