Add package.json configuration for JSPM support

Add package support for the [JSPM Module Loader](http://jspm.io)

## Problem
The package's main property points to source files instead of the distributable. When consuming this library with something like jspm we want to be using `dist/react-router.js`

## Solution
Fortunately jspm can utilize jspm-specific configuration so that a library doesn't have to alter their main property if they don't want to. See [Configuring Packages for jspm](https://github.com/jspm/registry/wiki/Configuring-Packages-for-jspm) for more information on the specifics.