Add src/ directory to webpack resolve root config

[andersonvom]

Currently, all imports in the code need to be relative to the current path, making it harder to refactor the code base as it's a pain to have to change all import paths, figuring out how many ../.. are needed.

Adding the src/ directory to webpack's resolve.root config allows developers to go from something like this:

import Header from '../../components/Header';
import TransactionForm from '../TransactionForm';
import * as AppActions from '../../actions';

to something like this:

import Header from 'components/Header';
import TransactionForm from 'containers/TransactionForm';
import * as AppActions from 'actions';

This change is also backwards compatible, so relative imports still work.