Single-file uploader -- Allow user to change which file will be uploaded

If maxFiles is set to 1, you have one file already added, and you try to add one new file, replace the file already added with the new file instead of giving a "Please upload 1 file at a time." alert.
Merge pull request #69 from stu-blair/master

Fixed issue with single-file uploaders where one is unable to change what file is first added