8 support IS_READ_ONLY for Data and Group rows (closes bitfireAT/vcard4android#8) (#9)

* IS_READ_ONLY support for Data rows

* GROUP_IS_READ_ONLY support for AndroidGroup

* introduce default parameter to leave tests unchanged

* add test to check whether the read-only flag is set for groups

* add test for DataRowBuilder setting IS_READ_ONLY

* remove default param and edit all the constructor calls in tests

* add KDoc params to DataRowBuilder class