Feature/generate index js

[TateB]

Created a script that generates the index.js file based on the built contract ABIs, and added to the build script.

[jefflau]

This adds every Solidity contract into index.js, but I'm not sure if we want to expose every contract, such as pure testing contracts:

const TestNameEncoder = require('./build/contracts/TestNameEncoder')
const TestRRUtils = require('./build/contracts/TestRRUtils')
const TestRegistrar = require('./build/contracts/TestRegistrar')
const TestResolver = require('./build/contracts/TestResolver')
const TextResolver = require('./build/contracts/TextResolver')
const UniversalResolver = require('./build/contracts/UniversalResolver')
const UpgradedNameWrapperMock = require('./build/contracts/UpgradedNameWrapperMock')

The ones with Mock/Test probably don't need to be exported and probably can be omitted from the npm package to save on size.

Also NameGriefer.sol is an example contract to test for a vulnerability on the NameWrapper. I guess we could rename it to Test and regex out anything that starts with Test