Improve rendering time using shouldComponentUpdate and PureComponent

[gpbl]

Analyzing #374 I've found we can optimize the re-rending of the components by using shouldComponentUpdate or PureComponent.

• Caption component – should be using shouldComponentUpdate to avoid useless re-rendering. Should update only if locale and date's year/month change.
• NavBar component – can extend PureComponent
• Weekday component – can extend PureComponent
• Weekdays component – can extend PureComponent
• Day component – should be using shouldComponentUpdate to avoid useless re-rendering. Should update only if the day's modifiers change.

The Month component uses its child as function to render Day, this is a weird pattern and makes optimization harder:

• move renderDayInMonth and renderDay method from DayPicker to Month
• Month component should be using shouldComponentUpdate to avoid useless re-rendering. Should update if modifiers change.

[gpbl]

Implementing shouldComponentUpdate in Month is more difficult as the modifiers may have different values which are harder to compare. Anyway, after the optimization above we can see less wasted time on re-rendering components.

This is more visible when rendering multiple months, e.g. with numberOfMonths={24} and selecting one day in the calendar:

Before we see a wasted re-rendering of 874 out of 875 days, and for the other components like Caption and Weekdays:

After no wasted rendering for days and other components:

Optimization is less visible when selecting one day in a one-month calendar, e.g. before:

and after (no wasted time re-rendering the calendar):