Sandboxing, custom folder icons, error handling, bug fixes

[crahan]

New features:

• The ability to restrict file browsing to a sandbox_path folder has finally been added! (Fixes #35)
• Filenames can not contain path separator characters or parent folder strings (i.e., '..')
• use_dir_icons has been replaced with dir_icon which allows for customizing the folder icon (Fixes #22)
• dir_icon_append can now be used to put the folder icon before or after the folder name
• Better error handling with ParentPathError, InvalidPathError, and InvalidFileNameError

Bug fixes:

• Better and more consistent handling of Windows drive letters and paths
• Fix bug where resetting the filechooser would not reenable the select/change button
• Properly handle folder permission errors by raising a warning