Allow configuration of the Parsoid host

Fixes #48 

Currently, there's not a convenient way to test this, since the mechanism for loading a configuration file lives in *./server.js*, which isn't used during testing.

Before we merge this, let's consider how to unify configuration loading so it's used both by `node server` and by mocha.
Is this a reasonable pattern?  The idea is that a handler exports one of two things:

1. The handler instance directly, with a `.paths` field
1. A function that takes in the `conf` variable, which it uses to return the handler instance