gitweb: Enable running gitweb as PSGI app, via CGI::Emulate::PSGI

gitweb: Enable running gitweb as PSGI app, via CGI::Emulate::PSGI

This commit makes it possible to run gitweb as a PSGI application
(see http://plackperl.org), using any of PSGI web servers.  This
includes ability to run gitweb from command line via Plack::Runner
module; just run "./gitweb.cgi --psgi" and point web browser
to http://0:5000

Gitweb uses CGI mode by default; to trigger PSGI mode one must either
pass `--psgi` or `--plackup` option to gitweb script, or the script
must be run with *.psgi filename (either copy/rename script, or make
symlink ending with *.psgi e.g. gitweb.psgi).

Besides running gitweb itself, converting its run_request() from CGI
to PSGI via CGI::Emulate::PSGI, gitweb as PSGI app also serves static
files: scripts, stylesheets and images via Plack::Middleware::Static.
Currently it assumes that positions and URLs of those assets are left
at their default values, namely that URLs end with "/static/<file>",
and that files can be found in "static/" directory relative to the
gitweb script itself.  This assumption should be relaxed in future.

Currently "git instaweb --httpd=plackup" doesn't (yet!) use this newly
introduced feature: it is left for future commit.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>