Improve variable name 'filename' to be fifoFile.

'filename' is extremely generic.
The updated name 'fifoFile' communicates clearly that the file in
question is the the FIFO created to communicate with phantom.
Add a comment to clarify that file is run by Phantom

In particular, it's useful to note that it has it's own 'fs' module,
whose API differs from that of the core node.js module.
Document the internal API of using "!" to indicate a rendering failure.
Improve the name of the 'st' argument.

Before it wasn't clear what 'st' argument was for.

I updated it to be requestStatus, which makes it a little easier to
understand what the code is doing.
Merge branch 'explain-the-bang' of https://github.com/markstos/phantom-render-stream into markstos-explain-the-bang

Conflicts:
	phantom-process.js
Merge branch 'markstos-explain-the-bang'
fix indentation