Setting Locale in DatePicker Components

[SimonPavlik]

Introducing a new language property used to set locale in DatePickerSingle and DatePickerRange components.

• Passing the default parameter auto will pick the browser's default language to set the locale.
• For example: language='en' sets the default locale of these components globally to English.
• Caveat: The same locale needs to be set for all DatePicker components located on the same page
• This limitation is caused by the fact that the locale is being set globally using Moment.js - a peer dependency of react-dates.

This PR resolves #579 and https://community.plot.ly/t/changing-calendars-langage-to-french/19983.

[roskoN]

Hey,

I do have a great interest in moving this forward.

Is there any way I could help? Are the unit-tests the only thing that is blocking this PR?

Thanks!