Make the pip() function more usable

[dloscutoff]

In theory, running pip("code", ["arglist"]) from within Python code should work nicely; and indeed, it does work nicely from within IDLE. But in any other context (Python's built-in REPL, other Python code), the fact that the function calls sys.exit(1) when it runs into a fatal error is unexpected and a bit tricky to deal with. A better approach would be to raise a normal exception (maybe FatalError) and let the caller handle it. When running pip.py as the main program, one approach might be to catch the error in the if __name__ == "__main__" section and do sys.exit(1) there.

In a similar vein, sys.exit(0) could instead be return, which would play a lot nicer with anyone trying to call pip() from their own code.

Finally, there are some parts of the current pip() function that ought to be refactored and called something else. The argparse code could probably go in its own function, for starters; and any code that assumes we're coming from a command-line invocation probably belongs outside of pip(), which might be called from some other context.

[dloscutoff]

Fixed the sys.exit parts. The refactoring isn't as important, so it has been moved to its own issue (#69).