Revert "rename source -> filename"

This reverts commit 190ba98e4414c39a22c96d8fef4af98d82f413fd.

Conflicts:
	History.md
	Readme.md
	test/css-parse.js
Get tests working again
Update readme
Add node.position.content containing source string
Merge pull request #74 from reworkcss/source-map-fixes

Source map fixes