Update getFileSizeString()

The "setprecision()" call used to do nothing because we printed integers (converts to doubles now).
Manually expanding multiplication only removes readability, the compiler will do these simple optimizations.
"else if" is exponentially more clean than the old "if"s.
Also fixed (eg "3.100 GB") looks messier than without fixed (eg "3.1 GB")
Merge pull request #49 from iitalics/patch-1

Update getFileSizeString()