Use App.base when there are no requests to generate URL's

Setting App.fullBaseUrl to a non-host value causes issues generating
URL's in a web context. However, not including the base path generates
incorrect URL's in a CLI context. This change allows the use of App.base
to populate the base path which is used as a fallback when there are no
requests, or when the base path is empty.

Refs #2931
Merge pull request #1718 from cakephp/issue-2931

Use App.base when there are no requests to generate URL's

Fixes #2931