Add decoding for "zTXt" chunks

[s373r]

Stored information takes advantage of compression. Decompression is needed before print.

4.2.3.2. zTXt Compressed textual data
+--------------------+-------------------------------+
| Keyword:           | 1-79 bytes (character string) |
| Null separator     | 1 byte                        |
| Compression method | 1 byte                        |
| Compressed text    | n bytes                       |
+--------------------+-------------------------------+
(c) http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html