Merge pull request #3 from carlpett/master

Make FileSystem instances track their own current directory
Removed circular dependency between FileSystem and FileSystemImpl by letting IInternalFileSystem interface to work with strings instead of IRootedFileSystemItems.
Moved DirectoryDescribingSpecialFolder from IFileSystem to IFileSystemExtensions. SpecialFolder is not something that fs4net should handle.
0.6: Bug fix: let the file system keep track of current directory (API behavior change); Removed support for SpecialFolders from IFileSystem and fixed backwards compability by adding an extension method. Updated copyright date.
Ignore tests that assumes that a drive does not exist on the CI since I don't know what drives exist there.
Ignore tests that assumes that a drive does not exist on the CI since I don't know what drives exist there.