Make easier to work with in node

In master, usage in node is super painful:

```coffee
global.esprima = require('polymer-expressions/third_party/esprima/esprima').esprima
{PolymerExpressions} = require 'polymer-expressions/src/polymer-expressions'
```

This fixes by

* Pointing main at ./src/polymer-expressions.js
* Uses `require` when it's available

With this PR, it's

```coffee
{PolymerExpressions} = require 'polymer-expressions'
```