Include javadoc comments on package declarations

The java standard allows for javadoc comments, in addition to annotations,
before the package identifier in the file package-info.java.

http://docs.oracle.com/javase/specs/jls/se7/html/jls-7.html#jls-7.4

This commit looks for javadoc in addition to annotations preceding the
package declaration, however it does not limit the search to just files
name package-info.java.
Using `parse_modifiers` here would (incorrectly) allow modifiers on package declarations. I would recommend just following the same pattern `parse_modifiers` and `parse_enum_constant` use for extracting Javadoc,

```python
next_token = self.tokens.look()
if next_token:
    javadoc = next_token.javadoc
```