Release GIL during initialization

[ochedru]

This helps mitigate issue #44 this way:

• trigger import of parse_address in a dedicated thread.
• the PR releases the GIL during the initialization so the other threads are not blocked in python code and can proceed with other tasks.
• if another thread imports parse_address, it will wait for the first import to complete.

Example:

import threading

def _import_postal():
    from postal.parser import parse_address

threading.Thread(name='import-postal', target=_import_postal, daemon=True).start()

# do something useful until import completes

[brandonrobertz]

What's the status on this PR? Does it need more testing or were there issues with it?

[ochedru]

FWIW we've been using this in production for some time now.