[Snap!] negative numbers trigger an error in the compiler

[Bernat Romagosa]

( ( 3 ) + ( -1 ) ) triggers the following error:

ucompile.js:177 
Uncaught Error: Argument does not fit in 24 bits
    at Compiler.addBytesForInstruction (ucompile.js:177)
    at ucompile.js:156
    at Array.forEach (<anonymous>)
    at Compiler.bytesFor (ucompile.js:154)
    at ThreadManager.startProcess (upatch.js:966)
    at ThreadManager.toggleProcess (threads.js:188)
    at ReporterBlockMorph.BlockMorph.mouseClickLeft (blocks.js:3807)
    at ReporterBlockMorph.mouseClickLeft (blocks.js:5234)
    at HandMorph.processMouseUp (morphic.js:10864)
    at HTMLCanvasElement.<anonymous> (morphic.js:11515)

[Bernat Romagosa]

fixes #5 (negative numbers)

→ <<cset 2f4ef5f8756b>>