patch, 3 partsgit-help: add new options -w (for web) and -i (for info)

Hi,

Here are the changes since my last RFC/PATCH:

[PATCH 1/3] git-help: add -i|--info option to display info page:

- info is now in a separate patch

[PATCH 2/3] git-help: add -w|--web option to display html man page in a browser.

- git-html-help.sh has been renamed git-browse-help.sh
- use web.browser config option instead of web.tool
- look up for the following documentation directories:
	"$PREFIX/share/doc/git-doc"
	"$PREFIX/share/doc/git-core-$GIT_VERSION"
not only the first one 
- add links, lynx and dillo has possible browsers
- git-browse-help.sh accept both -b|--browser and -t|--tool to specify
a browser (to be consistent with both git-mergetool and git-instaweb)
- improved launching of firefox|iceweasel and konqueror

[PATCH 3/3] Use {web,instaweb,help}.browser config options.

- improve consistent use of config options between git-browse-help and
git-instaweb

Thanks for your reviews, Christian.