Merge pull request #890 from drush-ops/library-caching

Enables caching for libraries.

Signed-off-by: Jonathan Hedstrom <jhedstrom@gmail.com>

Conflicts:
	commands/make/make.drush.inc
Merge pull request #907 from drush-ops/library-caching

Merge pull request #890 from drush-ops/library-caching