API: Added Executor::createLocal() and Executor::createRemote().

This API change lets processes be passed around to other parts of the system that may wish to manipulate their input & output streams.

We also add FilesystemEntity::createProcess().
REFACTOR: Introduce SSPakFile::writeFileFromProcess()

This new function let's SSPakFile take care of how it streams a process into the pakfile.  Obviously, it required Webroot::createProcess() to be available first.
NEW: Allow the creation of non-executable .sspak files.

Now, you can create a file called '.sspak.phar' and it will be executable, or '.sspak' and it won't be.