implemented also SetCurrentDirectory()
If File not exists, GetLastAccessTime, GetLastWriteTime and GetCreateTime should return default value of 1601-01-01 (from MSDN: If the file described in the path parameter does not exist, this method returns 12:00 midnight, January 1, 1601 A.D. (C.E.) Coordinated Universal Time (UTC).)
MockFile.Move can now throw exceptions (ArgumentNullException, IOException & FileNotFoundException)
Formatting
MockFile.Move handles now invalid filenames (blank, empty, null) with correct exceptions.
When Filenames contains invalid chars (Path.GetInvalidPathChars) then a ArgumentException should be thrown.
cleaned up tests for Exceptions - thanks to https://github.com/aklg :)

Changed the messages of exceptions to be like system.
Merge pull request #40 from srasch/master

Added SetCurrentDirectory and corrected MockFile.Move for errors