Rewrite CSS url() and font-family output logic.

The new logic is as follows:

* Given a URL to insert into url(), check that it is properly URL
  encoded (in particular, a doublequote and backslash never occurs
  within it) and then place it as url("http://example.com").

* Given a font name, if it is strictly alphanumeric, it is safe to omit
  quotes. Otherwise, wrap in double quotes and replace '"' with '\22 '
  (note trailing space) and '\' with '\5C ' (ditto).

We introduce expandCSSEscape() which is a hack for common parsing
idioms in CSS; this means that CSS escapes are now recognized inside
URLs as well as unquoted font names.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

tests/HTMLPurifier/HTMLT/shift-jis-preserve-yen.htmlt

@@ -4,5 +4,5 @@ if (!function_exists('iconv')) return true;
 Core.Encoding = "Shift_JIS"
 Core.EscapeNonASCIICharacters = true
 --HTML--
-<b style="font-family:'&#165;';">111</b>
+<b style="font-family:&quot;&#165;&quot;;">111</b>
 --# vim: et sw=4 sts=4