gitweb: Create Gitweb::Util module

gitweb: Create Gitweb::Util module

Create a Gitweb::Util module, which is meant to contain internal
utilities used by gitweb.  Currently it includes all the
quoting/unquoting and escaping subroutines that are used by the
gitweb, HTML-aware string manipulation subroutines, and some file
and filesystem operations.

The intent is to include in Gitweb::Util subroutines and variables
that do not require git or gitweb knowledge.

Update gitweb/Makefile to install Gitweb::Util module alongside
gitweb.

This was sort of part of [unfinished] Google Summer of Code 2010
project "Splitting gitweb and developing write functionalities
(Integrated web client for git)" by Pavan Kumar Sunkara, mentored by
Christian Couder and co-mentored by Petr Baudis.

[This reduces size of gitweb.perl by around 5%]

Signed-off-by: Pavan Kumar Sunkara <pavan.sss1991@gmail.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>