Permission mode for windows *alongside* the Unix implementation

[thearian]

Fixed #40 and #37 by implementing a -somewhat- cross platform solution.

This issues were addressed before, in #38 but it is imaginably not precise in Unix.
(Since 755 permission mode is not quite the readonly, but windows users just don't have much of a choice)

So I made a OS dependent function to check a file metadata permission: is_readable.

• In Unix: This function will be checking the 755 mode (aka the previous method)
• In Windows: It will return whether the directory is readonly or not