Format code consistently, add STYLE.md

First part of #5.

Code style is as [Felix's Node.js Style Guide](http://nodeguide.com/style.html), with a few [amendments and additions](https://github.com/passcod/mtwitter/blob/code-style/STYLE.md) (see `STYLE.md`). It is also passed through [JS Hint](http://www.jshint.com/docs/). A number of style points are not respected yet, like variable naming, function length, nesting depth limitations, etc — these require more thorough and less systematic changes.

If you're writing code to be merged in this repo, please follow these. If you wish to argue for or against specific style points, please do so. This pull request will remain open for at least 3 days to allow proper visibility.

## JSHint

Options used:

```
node:     true
curly:    true
eqeqeq:   true
immed:    true
indent:   2
latedef:  true
noarg:    true
noempty:  true
nonew:    true
plusplus: true
quotmark: single
undef:    true
unused:   true
trailing: true
maxlen:   80
```

`/*global escape*/` is also needed in `twitter.js` as `node: true`
doesn't whitelist it.