Added S-expressions.
Fixing negative numbers in s-expression parser.

The check to see if the first byte is 0..9 was added for performance (to
avoid trying to parse every literal as a number) but - was forgotten.  I
could have added other literals like + there as well, but I think - is
the only one that's actually required for non-"weird" floats.
Merge pull request #233 from pythonesque/sexp

Added S-expressions.