gitweb.psgi: Use installed static files, if they are available

gitweb.psgi: Use installed static files, if they are available

When gitweb is running as PSGI app, it must itself take care of
serving static files: stylesheets, script, images that are required
to render gitweb output.

This commit makes gitweb (in PSGI mode) use installed static files
from $(gitwebstaticdir) if such directory exists.  Before this commit
gitweb served static files from 'static/' directory relative to
position of gitweb script itself (to __DIR__).

Note that mechanism of serving is slightly different: the one with
__DIR__ uses Plack::Middleware::Static, while the installdir one uses
URLMap-ped set of Plack::App::File.

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