Adding options object for p4 exec.
- Allows user to set P4CONFIG environment var among other things.
- Allows user to set impersonated user if running as root.
- Also updating behavior of callback, will call with stderr AND stdout
  if there's an error.
- If there's a file passed in stat, change directory to its containing
  folder before running stat, and run stat on the filename, not the path
  to it.
- "Fix" stderr for p4 fstat *, removing "no such file" errors.
0.0.6