Handle multiple author tags while parsing javadoc strings.

I ran into a slight problem with this module while using [javasphinx](https://github.com/bronto/javasphinx) (which  depends on it).

According to the [Javadocs specification](http://www.oracle.com/technetwork/java/javase/documentation/index-137868.html#@author):

> You can provide one @author tag, multiple @author tags, or no @author tags.

Currently, javalang will set the `author` value to the last author tag found. I would consider this a bug, since many java source files contain a list of author tags, and this list would be systematically suppressed by the current parser.

This patch fixes this problem, and allows multiple authors. I can also fix the downstream implementation in javasphinx, if that seems appropriate.