Added child elements of Day to the agenda

[andru]

This is a small pull request which adds a means to add content to a day element.

I'm not sure what the long-term plan for the agenda is, but this pull-request allows any content to be added to the agenda by adding children to the Day element, e.g

//enable dayAgenda for all days
<Day dayAgenda={true} />

//add some content to today's agenda
<Day date={moment()}>This text will be appended to the agenda div</Day>

[freiksenet]

Thanks!