Read program code from stdin when not filename is passed.

This allows HippyVM to read it's program code from stdin when no filename is passed, thus mimicking the reference PHP interpreter (and just about any other interpreter you can think of). I *think* this is required for a SimpleCGI/Apache setup too.

Since we cannot import stdin later (right?), we effectively treat it as an un-cachable file.

To test, I've used the pexpect module to open a pipe to an untranslated `hippy/main.py`, so we add a dependency upon pexpect. However, we could just as easily skip these tests if pexpect is not installed (I can do this as a subsequent commit).

What do you guys think?