Allow for basic customization of the receiver window

[killthebuddh4]

There's a few simple customizations we should allow, a few that have already been requested:

• custom font
• custom border-radius on the window
• custom positioning of the window

First thoughts about doing a custom font:

• receiver components should not have fonts set (they should just inherit them)
• users should be able to do something like #receiver * { font-family: 'My Custom Font'; }

First thoughts on border-radius:

• We can probably just pass in a className prop to the <Window /> component for now

For custom positioning:

• The receiver window component has a baked-in fixed position wrapper, this is no good.
• Users should be able to put the window wherever they want.
• The default configuration should just work. Being able to say "just add " to "" and you're done is great.
• Different positions will need different transitions. We could provide a set of position components that include nice transitions for the respective positions.

[killthebuddh4]

There's another ticket open for custom fonts, and I think if I implement that right now there will be tons of merge conflicts with #53, so I'm not going to do the custom fonts on this one.