v6.21.0. Improvement. Bugfix.

- v6.21.0 January 2, 2013
	- Cleanup focused around loading, parsing, and writing of files and
documents
	- Added
		- `DocPad::flowDocument`
		- `DocPad::loadDocument`
		- `exists` attribute on `File` model
	- Fixed
		- `Document::writeSource`