Make it possible to specify FitNesse version as configuration in the pom

[GoogleCodeExporter]

Because the plugin currently uses a couple of customised FitNesse bootstrap 
classes, some attention would have to be paid to how much the FitNesse code 
changes between their releases.

Not yet sure what the overall strategy would be. It might need an adaptor 
pattern for the bootstrap classes, or perhaps some kind of plugin internal dumb 
launch and relying on the FitNesse URL's staying constant.

Or we may be stuck with tying plugin versions down to FitNesse versions.

In any situation, it would be best to maintain an internal mapping of 
compatible versions so that the plugin could handle configurations more 
gracefully.

Original issue reported on code.google.com by stewart@javahelp.co.uk on 8 Jul 2014 at 12:06