allow using the 'a' flag as a mode for opening a GzipFile.  gzip
files, surprisingly enough, can be concatenated and then decompressed;
the effect is to concatenate the two chunks of data.

If we support it on writing, it should also be supported on reading.
This *wasn't* trivial, and required rearranging the code in the
reading path, particularly the _read() method.

Raise IOError instead of RuntimeError in two cases, 'Not a gzipped file'
and 'Unknown compression method'