Refactor into named functions, automatically choose between `serve` and `server`

This fixes #30 and refactors the plugin into named functions. Tested manually, seems to be working well (but you're right, we need tests).

@dannygarcia, what do you think of the refactor? Want to get your opinion before I merge. I'd like to get this in and roll a new npm release so I can release a new version of generator-jekyllrb this week.