* Added a utility that can be used to produce nice HTML pages from Nix

  build logs.  The program `log2xml' converts a Nix build log (read
  from standard input) into XML file that can then be converted to
  XHTML by the `log2html.xsl' stylesheet.  The CSS stylesheet
  `logfile.css' is necessary to make it look good.

  This is primarily useful if the log file has a *tree structure*,
  i.e., that sub-tasks such as the various phases of a build (unpack,
  configure, make, etc.) or recursive invocations of Make are
  represented as such.  While a log file is in principle an
  unstructured plain text file, builders can communicate this tree
  structure to `log2xml' by using escape sequences:

  - "\e[p" starts a new nesting level; the first line following the
    escape code is the header;

  - "\e[q" ends the current nesting level.

  The generic builder in nixpkgs (not yet committed) uses this.  It
  shouldn't be to hard to patch GNU Make to speak this protocol.

  Further improvements to the generated HTML pages are to allow
  collapsing/expanding of subtrees, and to abbreviate store paths (but
  to show the full path by hovering the mouse over it).