Implemeting help message for servo browser app using groups::opts module.

Signed-off-by: Adenilson Cavalcanti <a.cavalcanti@sisa.samsung.com>
Following suggestion in the merge request, reuse the parameter passed to the
function that has the application name (so in future, if the browser no longer
is named 'servo', the code will continue to work fine).

Signed-off-by: Adenilson Cavalcanti <a.cavalcanti@sisa.samsung.com>
auto merge of #1277 : Adenilson/servo/implementingHelp, r=pcwalton

Executing servo browser for the first time doesn't make evident that it already supports several different flags.

This patch implements a help message displaying the available options and a brief explanation on them, based on comments in source code (opts.rs).

I implemented both short hand/long hand options using getopts::groups.