Implement alias resources. Key features:

Implement alias resources. Key features:
- configured at the context level in the same way as the other resource related attributes
- maps paths to directories or WAR files (single files not supported)

Implementation notes:
- Correct results for getRealPath() required this to be pushed down to the BaseDirContext as the short-cuts previously used needed to take account of any aliases. This in turn meant an addition to the Context interface
- Thanks to Tim F. The configuration format is all his idea

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@784083 13f79535-47bb-0310-9956-ffa450edef68