String default values becomes "''"

[helje5]

This SQL:

  note          TEXT NOT NULL DEFAULT '',

becomes this in generated init:

    note: String = "''",

and this in the mapping:

    public let note = MappedColumn<Account, String>(
      externalName: "note",
      defaultValue: "''",
      keyPath: \Account.note
    )

[helje5]

Fixed, a larger set of defaultValue expression is now implemented.
String generation of current time based types still needs to be done, issue #34.