Say something if the template engine isn't available for theme

fixes #600
Fix :num_pages options to keep backwards-compatible
Using old ruby hash syntax
Restore backwards compatibility of #num_pages in #paginate

Conflicts:
	lib/kaminari/helpers/action_view_extension.rb
Use class method '.themes' in ViewGenerator#copy_or_fetch
Only generate template files when doing rails g kaminari:views ...

This basically reverts 12822c9ee127664896d52ba5713d7c0c8f47794d.

closes #623.