Restructure modules to simplify settings.py import

This change moves the receivers, handlers, senders, and matchers modules
under a new module at hydrogen.modules . These four modules are the ones
which should be imported from settings.py . Whereas previously the
import statement necessary to import these modules was:

    from hydrogen import receivers, handlers, senders, matchers

And had to be inside the BUILD_CHAINS function, now the import statement
is can be as simple as:

    from hydrogen.modules import *

And can be outside of the BUILD_CHAINS function.