Configurable client

This pull request introduces a configuration DSL typical of other API clients for setting up authorization. 

Basic authorization

```ruby
Trello.configure do |config|
  config.developer_public_key = TRELLO_DEVELOPER_PUBLIC_KEY
  config.member_token = TRELLO_MEMBER_TOKEN
end
```

2-legged OAuth authorization

```ruby
Trello.configure do |config|
  config.consumer_key = TRELLO_CONSUMER_KEY
  config.consumer_secret = TRELLO_CONSUMER_SECRET
  config.oauth_token = TRELLO_OAUTH_TOKEN
  config.oauth_token_secret = TRELLO_OAUTH_TOKEN_SECRET
end
```

3-legged OAuth authorization

```ruby
Trello.configure do |config|
  config.consumer_key    = TRELLO_CONSUMER_KEY
  config.consumer_secret = TRELLO_CONSUMER_SECRET
  config.return_url      = "http://your.site.com/path/to/receive/post"
  config.callback        = lambda { |request_token| DB.save(request_token.key, request_token.secret) }
end
```

The current convention of setting `OAuthCredential.consumer_credential` etc. is still supported though I'd like to see it deprecated.

I believe this approach would make it easier to get up and running and may be a good solution to the ideas suggested previously https://trello.com/c/zmmvPD3k.