Minor Changes to the Filesize calculation

The function `getFs($file, $hr)` now uses per OS specific functionality to get the correct
filesize.
The divisor for the filesize for MB has been adjusted to
divide by 1024*1024=1048576 instead of 1024000. (only makes a noticeable difference for bigger files)
Also added a Gigabyte divisor for even bigger files.

Added a parameter `$hr` (human readable) that defaults to true. if it is set to false the function will return the raw number of bytes. (useful if a download capability should be added)