Add adapter class type for Lotus::Model#adapter DSL

This allows us to register multiple adapter of same adapter type, for example

```ruby
adapter :sqlite3, type: :sql, SQLITE3_CONNECTION_URI
adapter :postgreql, type: :sql, POSTGRESQL_CONNECTION_URI
```
Merge branch 'master' of github.com:lotus/model into adapter-name
Merge pull request #74 from joneslee85/adapter-name

Add type for adapter class name inflection for DSL