build.xml has specific koan targets; remove_snippets no err msg on Linux

Hi Jim,

I made a few minor changes that I complained about in my thornydev how-to :-).  Namely, specific targets for each Koan in the ant build file and squashing the rogue error message from remove_snippets.sh.

I figured out that the problem with remove_snippets.sh is that sed looks to have different command line option nuances between Mac (what I'm guessing you develop on) and Linux/Cygwin.  I modified remove_snippets.sh to detect os and set the flag appropriately - it should still be Bourne shell compatible (no bash/perl).  I tested it on Xubuntu Linux and Mingw32 on Windows (I don't have cygwin configured to use Java correctly, so didn't test that).  If you like either of these changes, please test them on Mac, since I don't have access to one.

-Michael (@midpeter444)