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.

首先，在 package 或 imports 之前放置一個與 google-java-format 格式不同的授權標頭，以重現此問題。追蹤 formatter 如何處理這則開頭註解；當這類標頭維持不變，同時其餘 Java 原始碼完成格式化時，即可視為問題已解決。

由索引模型根據 Issue 內容生成。