Factor out Context1Model from Lzp3.
Add very simple PPM-D compressor/decompressor implementation.

Simple reference-counted storage of contexts, no context exclusion (yet).
Add field named MAGIC to the various de/compression objects.

This is the first step to using the magic number to make -t on decompression
optional. (Ie, the decompressor should read the magic number and use that
to select the appropriate decompressor.)