Tutorial code through ch. 3

Pretty ugly & an annoying limitation: all tokens must be space delimited
Tutorial code through ch. 4
Reorganized; no functional changes
Transitioned from custom lexer to text/scanner
Fixed bugs initially missed in lexer swap & misc cleanups
Added some basic tests of the JIT
Fixed bug with commas in function calls & cont'd renames
Added support for 'if' expressions
Added support for 'for' loops.
More substantive README.md