Add metadata endpoint and update to API v3 endpoints

[DannyBen]

NOTE:
Since Quandl has done changes to their API, and since this PR uses the new Quandl API - it may break for users who used the previous version. In previous versions of the API, some responses had a docs property, which is now called datasets, and other responses had the data property, which is not available as dataset->data

This PR:

• adds the getMeta() method which maps to Quandl's metadata endpoint (https://www.quandl.com/api/v3/datasets/%s/metadata.%s)
• updates all the API endpoints to use v3.

Reference: Quandl API introduction page