Jump to joystick read

The program was reinitializing variables every time you jumped back
to line 100.  So, I made the loop jump to the joystcik read (line 407)
instead.
Fixed joystick GOTO

If the joystick wasn't moved or pressed, it should loop back to the
joystick read line (407), not to line 100.
Fix dot placement formula

- The formulas on line 417 and 418 were incorrect
- Add comment above initialization section
- Place initial dot onto screen
- Add code to erase dot at previous location
Limit x and y bounds

- Fix joystick Y math
- Add bounds checking
- Renumber
- Remove extraneous comments
- Add line number to last 7 lines of .basic file that had REM statements
Merge pull request #1 from paulog/code-review

Code review