the big rewrite

Everything is new, and we're now much closer to the original Bundler.
Also included is the start of an API.

Most commands beneath 'install' are broken now.
add --version flag to cli
Reimplement `pbundle init`
remove dead code from cli.py
whitespace/formatting/version cleanup
update USAGE text, rename upgrade->update
fix replace_sys_path not working properly

We were changing the list we were iterating over, never a good idea.
reimplement exec/console

Note that these were run/py previously.
Don't write ourselves to lockfile