Add  openHours field to places

so we can specify some informations about opening hours.

Probably not the best solution, but enough to start with and figure out if we need more than that.
Fix #9
optionToHtml now works like optionalValueToHtml

it take a label and a option (boolean + string) and returns the html representation needed to build the marker's popup.
Rename optionalValueToHtml in optionalFieldToHtml
Merge pull request #20 from avernois/issue_9_add_openHours

Add  openHours field to places