Add comment with info on Amazon S3 regions
Update info and default value of CLI region option

Fix the descriptions of the regions and make the default value 's3'.
Fix regex used to detect JPGs

Correctly detect files in EXIF format as well as JPEG/JFIF files.
Add the S3File class
Use an instance of the S3File class in PUTObject

Basically replace variables in the PUTObject class with methods in the
S3File class.