Customize genSrcPath, genTestSrcPath in eclipse APT gradle plugin

[stephenjust]

In the eclipse APT plug in, generated sources by default get placed in .apt_generated.

If this is configurable, could you please update the plugin docs to describe how to do so? If this is not configurable, could you update the plugin to make it configurable and update the docs?

[nedtwigg]

We're not using this plugin anymore, but PR's are welcome.