* all: remove "u" string prefix

* util: remove unirange

Since Python 3.3, all builds are wide unicode compatible.

* unistring: remove support for narrow-unicode builds

which stopped being relevant with Python 3.3

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

* Fix an issue raised in the review -- don't raise None no matter what.
* Remove a few variables to make the control flow easier to understand

This is slightly different from the code posted inside the issue:
* Use finally to close the registry key no matter what
* Don't use break, but return directly on the first hit

* Remove Python 2 compatibility

* remove 2/3 shims in pygments.util
* update setup.py metadata

* Remove unneeded object inheritance.

* Remove unneeded future imports.