Support Hidden Volumes

When using the ramdisk for caches and stuff I thought it would be pretty nice to be able to hide it from the Finder sidebar, and save/open dialogs. So I started looking into adding such a feature.

##### Changes

- Add Checkbox in UI for making a Hidden Volume, in NewTmpDiskWindow class and xib

- The main change for Hidden Volume support is in TmpDiskManager.m on line 90 and below. I would like to get that command a bit shorter, but haven't been able to find a good way yet. However it seems to work fine so far.

- Support for saving the "hidden" property to the prefs, trying to emulate the way you did it for the "indexed" property

- Some general fixes to warnings due to potentially unsafe casts, also changed all MB calculations to
  use 1024*1024/512 everywhere, at least makes it consistent. But If we intend to emulate the way the Finder does
  it we may have to change it to 1000*1000/512 (I think).