Call configure.py from wscript

Whenever I add a new TOTP key to pebble-authenticator, I always forget to run configure.py.

This patch solves that issue by moving `configure.py`'s "make things happen" code into a function (`configure.generate()`) and adding a call to the function in wscript. `configure.py` will also check if it is being run as the main script and, if it is, will regenerate `configuration.h`.

Whenever `pebble build` is called, `configure.py` is imported and `configure.generate()` is called as a part of the `configure` step.