Reduce moment dependency by using native Date objects (breaking change)

[gpbl]

My plan to move away from moment.js didn't work, basically because I couldn't find any valid alternative for supporting multiple locales (see #2). I'd like, at least, to reduce the use of moment.js by doing the following:

• Rewrite CalendarUtils to work with javascript's Date. A well written utility doing similar things is this one from material-ui. This choice implies that the day picker will accept and return only Date objects.
• Move momentjs-related functions into an external util, so it will be easy to replace it as I find a better solution for replacing moment.js. Developers could provide their own utility using webpack's NormalModuleReplacementPlugin. Those functions are just to format dates to strings and to know the first day of the week.