Fix: #1299 - Add PYTHONSTARTUP support to pshell

This will load the `PYTHONSTARTUP` file.  It also handles injecting the globals in the `PYTHONSTARTUP` file into the interpreter.

There is at least one caveat wrt readline / tabbed completion.  In order to get tabbed completion working on modules that were imported once inside pshell, I had to add the following line to my `PYTHONSTARTUP` file:

``` python
readline.set_completer(rlcompleter.Completer(locals()).complete)
```

I am not sure why, but the normal python shell doesn't require that line to have tabbed completion work properly.