Merge pull request #1 from moment/develop

Fetch update
Merge pull request #2 from moment/develop

Merge commits from upstream
use YYYY-MM-DD for L in lang/zh-cn.js
Merge pull request #1346 from zenozeng/develop

use YYYY-MM-DD for L in lang/zh-cn.js
Fix zerofill for negative numbers
Add YYYYYY format for iso string
Use YYYYYY format in iso string for big and negative years
Fix tests
Merge pull request #1348 from ichernev/bugfix/YYYY-handle-negative

Fix leftZeroFill for negative numbers
Merge pull request #1349 from ichernev/feature/iso-long-year-format

toISOString supports long and negative years
Fix iso week parsing for small years

Apparently Date.UTC works only for years after 1900:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date

To form an iso date, from any given year you should use an iso formatted
string. That string doesn't need 'Z' in the end ...
Merge pull request #1350 from ichernev/bugfix/iso-week-parse-small-years

Fix iso week parsing for small years