Merge pull request #21 from KaiSmith/master

math_abs() now returns double.
Commented Kai's fix to issue #20 and updated lexer error messages/

Signed-off-by: Christian Gunderman <christian@sonofboss.cwru.edu>
Merge branch 'master' of github.com:gundermanc/gunderscript
Working Gunderscript shell
Minor shell fix