Allow Javadoc to reference imports

These style errors continue to creep in due to IDE misconfiguration so
we might as well stop checking for it.
> This commit allows Javadoc to reference imports, i.e., partially disables the check.

Ah, OK, thanks for explaining. [From the docs](http://checkstyle.sourceforge.net/config_imports.html) I think I now understand what this does.

Looks like something is [in the works](https://bugs.eclipse.org/bugs/show_bug.cgi?id=128661) for Eclipse, but +1 to merge this in the meantime. Thanks, @andrewgaul!