Add a named commonjs export for Big

[googol]

This enables the proper use of the module with es6 module syntax.

With just module.exports = Big, there is no proper way of importing it in a project using es6 module syntax. This is because the es6 import syntax doesn't allow direct access to the exports object.

This can be worked around when importing directly into a project that is transpiled into commonjs, but if you want to create a library that uses the es6 module syntax (to allow for tree shaking etc), those tricks are not usable anymore.

With the named export the old var Big = require('big.js') still works, but the es6 import { Big } from 'big.js' also works.

[MikeMcl]

Okay, thanks.