Markdown standardization - activators, addresses, and adjustments.
More standardization on markdown, more explicit explanation for the FlexiRate calculator.
Updates to the Calculators guide, including adding back in the bookmarks
Further markdown standardization
Deleted duplicate, inferior payment guide
Normalized the orders guide
Removed the unused duplicate line_item guide
[getting started] Correct extensions link at the end

Fixes #122

Conflicts:
	content/developer/tutorials/getting_started_tutorial.md
	lib/resources.rb
Moved triple-ticks to the line above on preferences tutorial
fixed text in checkout.md. grammar and path fix [Fixes #126]

' to to ' changed to ' to '
path reference to checkout directory in core changed to frontend

	modified:   content/developer/customization/checkout.md
Normalized the payments guide
Normalizing preferences tutorial, more changes to markdown standards (WIP)
Typo fix for Addresses documentation.
Merge pull request #127 from onedanshow/master

Typo fix for Addresses documentation
[typo] replaced newline to separate docs/code
Variant responses will now include a description
Add display_single_amount, total_amount and display_total amount fields to line item responses
Make line item responses consistent with their Ruby counterparts
Fixed nanoc compile issue
Add display_item_total, total_quantity and token to orders API responses