sequenceModel instantiation should validate stickyEnds (and sequence) if present

[AJamesPhillips]

If a new sequence is made:

var attributes = {
  sequence: 'AT',
  stickyEnds: {
    start: {
      sequence: 'GG',
      offset: 1,
      size: 1,
    }
  }
};
new SequenceModel(attributes);

... then the attributes are invalid, and the sequenceModel should fail to instantiate. Either the sequence should be GGAT given the stickyEnd start sequence of GG, or the stickyEnd sequence should be AT.