Fix #886. Fatal error: Call to undefined function drush_module_exists() on test-run
Fix #879. Cater to older versions of Drush on a remote server when doing a user-login
Minor - reference the affected version, not the one before.
Change http to https. Progress on #894.
Fix reference to unknown variable in release_info engine.
Allow engines to live in lib/. Clean up, remove unneeded complexity.
Merge pull request #900 from jonhattan/engines-in-lib

Allow engines to live in lib/
Fix reference to unknown variable in release_info engine (2nd take).
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()`.