Automatic table name generation

[groue]

This PR brings a default implementation for the TableRecord.databaseTableName property:

struct Place: TableRecord { }
Place.databaseTableName // "place"

This default name follows the GRDB3 database schema recommendation: generated table name are singular, camel-cased, and look like a Swift identifier:

• Place: place
• Country: country
• PostalAddress: postalAddress
• HTTPRequest: httpRequest
• TOEFL: toefl

Due to a limitation of the Swift compiler, Record subclasses must keep on explicitly overriding databaseTableName:

class Place: Record {
    override var databaseTableName: String {
        return "place"
    }
}