Changes so pith can work on ruby 1.9.2

Hello Mike
I was getting errors running pith on ruby 1.9.2 (Psych::SyntaxError is undefined) within 'input.rb'
So I have changed to SyntaxError

In trying to get the pith dev enviroment installed, I ran into some known issues in [installing 'rdiscount' on 1.9.2](http://dafoster.net/projects/rdiscount/), so i've swapped it out in favour of redcarpet (another markdown parser), which I admit isn't as fully featured.

I've haven't written any extra specs, as the test suites are now passing on 1.9.2 and 2.0.0

I'm happy to accept that you don't wish to support 1.9.2, if that is the case can we add a note in the readme saying there are issues in running pith on 1.9.2

Thanks
Dougal