Fix a couple of issues with JDK7 javadoc style by using a slightly cu…

…stomized version of the default stylesheet in Guava. Specific issues this fixes:

- Huge text with a slightly misaligned first line in <pre>{@code} blocks in Guava javadoc (for example, see http://docs.guava-libraries.googlecode.com/git-history/v14.0.1/javadoc/com/google/common/io/Closer.html). Both <pre> and <code> were increasing the font size. This changes the <code> block to inherit the font-size of the <pre> block.
- No blank line before <pre> blocks in class-level javadoc. A CSS class rule for a div surrounding the class level javadoc set <pre>'s margin-top to 0.
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