Fixed error when application.conf includes no settings for scalikejdbc.global

- com.typesafe.config.Config#get* doesn't return null when configuration key is not present. It throws exception.
- Divided DBs and TypesafeConfigReader into small traits to read different configuration files.
Fliped the type names to be natural. TypesafeConfig <-> Config.
Added documents
Rename StandardConfig to StandardTypesafeConfig
Merge pull request #75 from tototoshi/fix-load-global-settings-error

Fixed error when application.conf includes no settings for scalikejdbc.global