add `reports` attribute to findbugs.xml.

This commit will add `reports` attribute to each of `<Detector>` element in `findbugs.xml`.

Please see [findbugs.xml which is packaged into findbugs.jar](https://code.google.com/p/findbugs/source/browse/findbugs/etc/findbugs.xml?r=16b42c7e45c326468c1a572885fbcf117a157eea#269) to know how this attribute should be used.