css: clean up documentation
commands: add a 'script' command

Similar to the 'css' command, but for JavaScript and only available in
WebKit2 >= 2.5.1.
commands: hook up message handlers from the page
docs: fix replacement commands for inject_* variables
fixup! commands: add a 'script' command
css: make the location and level optional
make: depend on uzbl-browser from install-uzbl-browser

The build failed with a "make install" because of this.