Datatable addition

[RaymondMcT]

Wanted to give a bit of a boost to the Quandl Julia package to keep it in line with the other packages we officially support. The main addition was the Quandl.datatable function, it interfaces with our datatables api, more and more data on Quandl is only accessible through that route now.

I made some other cosmetic changes to facilitate the adding of that function but I do believe I've maintained backwards compatibility.

I'm not too deep into Julia and package maintaining so there's a couple notable omissions.

• Version update (don't know where that goes)
• Tests (not familiar with the framework + mocking in Julia)

Let me know if you'd like me to make some updates to the docs with some example calls.