Added stopAtStopCodons

Implements enhancement suggested in issue #137
Added stopAtStopCodons option to RNAToAminoAcidTranslator, retaining existing behaviour.
Updated TranscriptionEngine#Builder with appropriate methods to use setting
Added a test case to TranslationTest to test that if this flag is set, translation stops. (The negative test is tested for by an existing test, which also therefore tests for the default to retain existing behaviour)