Fixes for webpack setup docs.

[simonbuchan]

Re-creating #117 after early merge was reverted, added fixes and improvements, rebased and squashed.

This should work and be mergable, but really should also update the usage sections as the AWSCognito usage will not work in webpack (by default - eg. you could use ProvidePlugin) - see original description below.

Also wondering if we should recommend transform-loader for aws-sdk instead to make it work the same as browserify? Should have the same result.

Original PR

Follow up to #108 updating usage docs.

Need to know what should happen to the usage, since it could be different in different environments. I'm currently thinking changing them to consistently use the new UMD root namespace AmazonCognitoIdentity instead of AWSCognito.CognitoIdentityServiceProvider (added as it needs to export something, and it replaces it - but the old still works due to ./enhance.js), e.g.

var userPool = new AmazonCognitoIdentity.CognitoUserPool(poolData);`

Then we just need to add a section to webpack setup that you need to add at the top:

var AmazonCognitoIdentity = require('amazon-cognito-identity-js');

I'd also like to gather some feedback before this gets merged on if I've missed steps, provide not enough (or too much!) explanation, should it include babel, etc...

I feel this doc should be split up into multiple files, but probably better in a separate PR.