add is_default method and write proper constructor
add is_default method and write proper constructor

dbolser/ensembl-variation,willmclaren/ensembl-variation,dbolser-ebi/ensembl-variation,Ensembl/ensembl-variation,Ensembl/ensembl-variation,dbolser/ensembl-variation,willmclaren/ensembl-variation,willmclaren/ensembl-variation,Ensembl/ensembl-variation,dbolser-ebi/ensembl-variation,dbolser/ensembl-variation,Ensembl/ensembl-variation,dbolser/ensembl-variation,Ensembl/ensembl-variation,willmclaren/ensembl-variation,willmclaren/ensembl-variation,dbolser-ebi/ensembl-variation