[WIP] Instantiate Adapter from Adapter config

Okay, so we have `Lotus::Model::Config::Adapter` to store the adapter settings. 

This PR is to add `load!` method to instantiate the adapter from the config object

For example:


```ruby
sql_config = Lotus::Model::Config.new(:sql, 'postgres://url')
adapter = sql_config.load(mapper) # you must provide a mapper object
# => [Lotus::Model::Adapters::SqlAdater instance]
```
@joneslee85 @robyurkowski I'm a fan of RSpec's `context`, but not of changing MiniTest ;)