Make sure completing-read-function is never set to nil

If "ido-ubiquitous-fallback-completing-read-function" is nil, just use
"completing-read-default".
Untabify
Eliminate inappropriate evaluation in macro

The override argument to ido-ubiquitous-with-override was being
evaluated at macro-expansion time instead of runtime.
Allow overrides to force ido when collection is a function

Fixes #36.
Enable ido in find-library

Fixes #28.