Don't reformat 'javadoc' comments that appear before package/imports, to avoid mangling license headers

The use case is license headers added by, for instance, maven [license](http://www.mojohaus.org/license-maven-plugin/) [plugins](https://github.com/mycila/license-maven-plugin). These headers are not by default formatted in the way that google-java-format requires, and since neither (for good reasons - neither code styles nor licenses should be changed lightly) are very configurable, conflicts are hard to avoid. (The mycila license plugin allows configuration, and with a bit of effort, you can make it create license headers that don't get reformatted - I haven't succeeded to do that with the Codehaus plugin).

Creating this issue as a different one than #139, which is about javadoc comments.