RFR - JL add carrierwave

When registering a new account, users can now upload an image (gem: carrierwave) which will be automatically resized to 100x100 while maintaining aspect ratio.

Added test in the user_creates_account_spec.rb to account for photo uploads - the default photo to be used for testing is located at /app/assets/images/sushi.jpg

Also hooked up to Amazon S3 for image storage - we still need to figure out the credential stuff - I have not uploaded my secret keys (because they're secret...?)


Use the newer syntax for validations (also, do you want a presence validation on username?):

```ruby
validates :username, 
  presence: true,
  uniqueness: true
```