add additional type check for model
add additional type check for model

if the model is typeof number (unix timestamp) the bug https://github.com/zhaber/datetimepicker/issues/13 would also occur.
this check prevents said error.
zhaber/datetimepicker,xdimedrolx/angular-js-bootstrap-datetimepicker,zhaber/angular-js-bootstrap-datetimepicker,necroscope/angular-js-bootstrap-datetimepicker,zhaber/angular-js-bootstrap-datetimepicker,transGLUKator/angular-js-bootstrap-datetimepicker,zhaber/datetimepicker,forestjohnsonilm/angular-js-bootstrap-datetimepicker,xdimedrolx/angular-js-bootstrap-datetimepicker,necroscope/angular-js-bootstrap-datetimepicker,forestjohnsonilm/angular-js-bootstrap-datetimepicker,transGLUKator/angular-js-bootstrap-datetimepicker