Use a bigger left & right margin for the table

The reason is that otherwise, the "Fix me on GitHub" ribbon invisibly
overlaps the SciJava Common icon (well, whatever icon is on the top
right of the table) making it no longer clickable.
Rearrange order of icons

This adds separate Java 6, 7 and 8 javadoc links.

It also centers everything nice and evenly.
Consistently use tabs instead of spaces in the CSS