Add support for annotation method default values in Javadoc hovers

Java supports display of default value in hover; add similar support to Groovy.

![default-value](https://user-images.githubusercontent.com/18193802/34737068-98feeb36-f53a-11e7-8e6f-0d727cd6b3af.png)
