Let formatSize return a specific size

Extend NumberHelper->formatSize(): now we can specify a specific unit.
For instance: formatSize(1024) returns '1.00 KB',
but formatSize(1024, 'bytes') returns '1,024 bytes'
Use NumberHelper->formatSize when formatting file sizes
Merge pull request #1594 from mlocati/use-numberhelper-formatsize

Use NumberHelper->formatSize