NEW: Added inputStream and outputStream options to Executor.

These options let streams (such as from phar fopen commands) be used as the input and/or output
of commands.  This will be useful when switching to a Phar-based format for sspak files.
Moved from tar-based form to phar-based.

SSPak now uses Phar as its storage format.  This makes for more elegant self-extracting phars,
and simplifies some of the code for reading/writing from the files.

Right now, the self-extractor doesn't have any knowledge of itself, and there isn't any
way of making a non-self-extracting file.  But it's a good start.