instaweb: support for Ruby's WEBrick server

instaweb: support for Ruby's WEBrick server

running the webrick server with git requires Ruby and Ruby's YAML and
Webrick libraries (both of which come standard with Ruby). nice for
single-user standalone invocations.

the --httpd=webrick option generates a ruby script on the fly to read
httpd.conf options and invoke the web server via library call. this
script is placed in the .git/gitweb directory. it also generates a
shell script in a feeble attempt to invoke ruby in a portable manner,
which assumes that 'ruby' is in the user's $PATH.

Signed-off-by: Mike Dalessio <mike@csa.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>