This seems to break some themes which were not expecting Pygments to
change margins, and it doesn't look like it makes a difference for
standalone Pygments.

This removes the top/bottom padding changes, and only keeps left/right
padding, in the hope that this does not break all Sphinx themes.

With the previous changes, we started to emit one <pre> per line for
line numbers. This breaks for instance the Sphinx-RTD-Theme, which
expects the line numbers to be formatted the same way as the normal
content. This commit makes the following changes:

* Emit a single <pre> inside the linenos div
* Wrap individual lines into <span> as needed
* Update all tests
* Don't yield empty <span> elements when no style is specified

This also makes the .html test files look correct when looked at with a
browser, as there is no extra whitespace in them which needs stripping.

* Add font and background colors to Style

* Move all styles to get_style_defs, add tests

* Remove hardcoded styles, add special lineno style

* Add styles for special line numbers in tables

* Update noclasses documentation

* Refactor linenos elements and styles, add tests

* Update AUTHORS

* Fix multiple CSS prefixes, add tests