Fix label padding for large numbers

[brdu]

Add this code to test.js to reproduce

var s3 = []
for (var i = 0; i < width; i++)
    s3[i] = (2000 * Math.cos (i * ((Math.PI * 8) / width))) + 10000 // values range from 9000 to 11000
console.log (asciichart.plot (s3, { height: 10, width: 60 }))

[coveralls]

Coverage decreased (-3.2%) to 96.774% when pulling c1aef00 on brdu:bd_fix_labels into be5de29 on kroitor:master.

[kroitor]

Thank you very much for this commit! I'll merge it asap!