Fix capitalization inconsistencies of #calendar

This is a fix proposition for #1190

It introduces an `upperCaseFirstLetter` boolean option to `#calendar`:

```js
moment().calendar(true); // -> 'Today at 10:47 PM'
moment().calendar(false); // -> 'today at 10:47 PM'
```
This feature is tested and all `test/moment/*` pass.

This option is `true` by default for reverse-compatibility (`test/lang/en.js` passes without any modification):

```js
moment().calendar(); // -> 'Today at 10:47 PM'
```

**However** this fixes capitalization inconsistencies observed in languages where the first letter of day names is not upper-case. For instance in french:

```js
// now
moment().add("day", 4).calendar(); // "dimanche à 9:00" expected "Dimanche à 9:00"
moment().add("day", 1).calendar(); // "Demain à 9:00"

// with this fix
moment().add("day", 4).calendar(); // "Dimanche à 9:00"
moment().add("day", 1).calendar(); // "Demain à 9:00"
moment().add("day", 4).calendar(false); // "dimanche à 9:00"
moment().add("day", 1).calendar(false); // "demain à 9:00"
```
See `test/lang/fr.js`

### TODO
- adapt for other languages than french (and english)