Add README, implement KILL/NAME, use relative paths in tests

Add README.md for public release. Implement KILL and NAME statements
for file management. Change test programs to use relative paths
instead of /tmp/ and clean up temp files with KILL. Update .gitignore
for test artifacts.

tests/programs/save_load.bas

@@ -1,3 +1,4 @@
 10 REM SAVE test
 20 PRINT "Program saved"
-30 SAVE "/tmp/gwbasic_save_test.bas"
+30 SAVE "gwbasic_save_test.bas"
+40 KILL "gwbasic_save_test.bas"