Simple wrapper around h.r.download

Set to buildout['buildout']['parts-directory'] + name by default.
Used in 50% of the recipes.

Fix aac29aa6.
parts+= *is* *a* *nightmare*.

parts+= is a nightmare in buildout.

request recipe: Only append SLA parameter to the list if the key actually exists and is not empty string.

Conflicts:
	component/imagemagick/buildout.cfg
	component/libtiff/buildout.cfg
	component/postgresql/buildout.cfg
	setup.py

Conflicts:
	stack/lamp/buildout.cfg