Add a new property to hold the current project's default target

Files:
modified: /ant/core/trunk/src/main/org/apache/tools/ant/MagicNames.java (try)
modified: /ant/core/trunk/src/main/org/apache/tools/ant/Project.java (try)
modified: /ant/core/trunk/docs/manual/using.html (try)
modified: /ant/core/trunk/WHATSNEW (try)
added: /ant/core/trunk/src/tests/antunit/core/magic-names-test.xml (try)