Fix issue 903 Cannot access protected propert when drush generate-makefile
https://github.com/drush-ops/drush/issues/903
README simplification. Also marked Drush5 as unsupported.
Remove references to www.drush.org.
Fix MSI link. Using an old Drush release to host it for now.
Enables caching for libraries.

- Resolves #824.
Simplify caching to be global unless explicitly disabled.
Removes unused call to `drush_get_engine()`.
Merge pull request #890 from drush-ops/library-caching

Enables caching for libraries.
Merge pull request #904 from drucloud/master

Fix issue #903 Cannot access protected propert when drush generate-makefile.
Fix regression in pm-updatestatus.