tools: Remove node.js pipbot implementation

We can now begin re-implementing in Python.

Issue #178.
build/tools: Add generic rule for Python scripts

Issue #178.
tools/pipbot: Begin re-implementation in Python

So far, we only have the version command working, but there's more to
come!

Issue #178.
tools/pipbot: Add 'help' command

Issue #178.
tools/pipbot: Add local file paths

Issue #178.
tools/pipbot: Refactor for better whitespace

Issue #178.
tools/pipbot: Add skeleton implementation of build()

Issue #178.