Add support for concatenated gzip streams

The gzip file format allows multiple compressed "members" in a single
file, simply concatenated together. The decompressed file is considered
to be the concetenation of the corresponding decompressed members.

Previously we just decompressed the first member (though we did at
least report the trailing data). Now by default we decompress all the
members. This behaviour can be controlled by the decompressAllMembers
field in the DecompressParams.

The test we use is that if the next two bytes are the two-byte gzip
header magic number then we expect another gzip member. If not then
we end the stream and report the trailing data. The implementation of
this is a bit tricky because of all the boundary conditions.

Author: dcoutts