Add support for NOW() as a default value for timestamp columns

[kilnerm]

Having the ability to use NOW() as a default value would allow users to implement an automatic created at column in their table definitions.

This work could be scoped with issue #117,

[taylorwmitchell]

This may be helpful as a temporary workaround. Note this is using a timestamp data type / MySQL.

/**
 When creating a Column, a default value can be specified. If a string
 is provided, the MySQLConnection.packType() method wraps the value
 with "\'". This makes sense if you are specifying a default value
 that is constant, but means the generated query for a current_timestamp
 default value doesn't work (MySQL wants current_timestamp, not \"current_timestamp\")
 
 The workaround is to provide a String(describing: _) value, which is done here.
 */
public enum DefaultValues {
    public static let currentTimestamp = CurrentTimestamp()
    public static let updatingTimestamp = UpdatingCurrentTimestamp()
}

public class CurrentTimestamp: CustomStringConvertible {
    public var description = "CURRENT_TIMESTAMP"
}

public class UpdatingCurrentTimestamp: CustomStringConvertible {
    public var description = "CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP"
}