Display omniauth provider when linking account.

There was no provider param, which left the flash message when connecting an
account a little empty. This adjusts the flash message to use the provider from
the omniauth hash.
Allow Supermarket to use S3

The current community cookbooks are hosted on S3 so Supermarket
must be able to support S3 for cookbook storage. Now if both a S3 bucket
is set paperclip will be configured to use S3 otherwise it will use
local storage. This is done so the application is not dependent upon S3.
Merge pull request #281 from opscode/s3_configuration

Allow Supermarket to use S3
Check for Supermarket bucket config using string keys

Magiconf uses string keys not symbol keys so this changes the
CookbookVersion tarball logic to account for that so it works.
Merge pull request #299 from opscode/symbols_are_not_strings

Check for Supermarket bucket config using string keys
Merge pull request #284 from opscode/account_flash

Display omniauth provider in flash when linking account.
Flesh out the content on the welcome page.

This is not the final design and content of the Welcome page. It is an attempt
to make it more useful by adding in relevant content that may be useful to new
users and those viewing the application.

- Add three columns of content, for explore, learn and share.
- Add Sign Up button to the welcome page.