Rename Archive::TarStream to Archive::TarReader, save -> extract

After a code review we decided that TarStream was a bad name, as this reads
a stream containing tar formatted data.

We considered Archive::Reader with a :tar argument to make the data
format explicit, but I suspect we're never going to develop a further
set of modules. If we do then this class can be used by an Archive::Reader
factory.

The save method was renamed to extract, as it extracts the file from the
stream.