Fix initial value for apiTimeDiff

Fixes "400 Invalid X-Ovh-Timestamp header" because of `this.apiTimeDiff === null` being false (at this point, it is `undefined` and not `null`), and therefore never properly initialized.
Merge pull request #1 from naholyr/patch-1

Fix initial value for apiTimeDiff
Include auth API for /auth/time if usedApi parameter is defined
Fix duplicate call of warning function
Version 1.0.1
Add a Bitdeli badge to README
Merge pull request #2 from bitdeli-chef/master

Add a Bitdeli Badge to README