Adjust javadoc.css and <pre> with class language-groovy to unified DOM
structure.

Javadoc <pre> element now contains <code> element to have the same DOM
structure as code samples in manual.
Then padding for <pre> is not needed. It is set on the <code> level.
Font size change is not needed on <pre> element too. It is set on the
<code> level.
Merge pull request #528 from zedar/master

Adjust javadoc.css and <pre> with class language-groovy to unified DOM