Annotation index: avoid iterating the same URL twice

When a URLClassLoader has a class path element that the parent class
loader also has, ClassLoader#getResources() will list it twice. Work
around that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Annotation index: Add a unit test for repeated class path elements

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #28 from scijava/repeated-annotations

Fix annotations reported multiple times

When a class loader lists the same element twice in its class path, or
one of its parent class loaders does, ClassLoader#getResources() will
report matching files multiple times, too.

This change works around that behavior by keeping track what class path
elements we already looked at and skipping them when encountering them
again.