Features/fitnesse plugin fix fitnesse server start

This plugin looks at standard outuput to know if Fitnesse server is started.

Last version of Fitnesse doesn't write on stdout when the server start.
(since https://github.com/unclebob/fitnesse/commit/91e93be - commit date: 13 nov 2013 - impacted version  20140201 &  20140418)

I've replaced this check by verify the target Fitnesse URL (do a HTTP GET on it).