Features/fitnesse plugin fix fitnesse server start

(sorry for the last one, I have fixed a misspelling)

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).