Create a download() method for downloading market data

[chiraganand]

MarketData.jl provides the function for downloading market data from yahoo and other sources but the primary method MarketData.yahoo() accepts only a single ticker (issue JuliaQuant/MarketData.jl#72). For the download function in TSx, I would want to have something like:

startdate = DateTime(2012, 10, 1)
options = DownloadOptions(src = "yahoo", period1 = startdate, join = JoinBoth)
download(symbols::AbstractVector{String}, options; on="timestamps")::TS

This should join all the tickers together using JoinBoth strategy on timestamps column and return a single TS object.

[chiraganand]

Join of multiple objects need to be allowed (issue #40) before download can work properly.

[ayushpatnaikgit]

I don't think this feature should be a part of TimeFrames. We need to find hard and push it to MarketData.jl

[ayushpatnaikgit]

@ajaynshah I think we had decided not to put this function in the package. We need to do a PR to MarketData.jl. Shall we close this issue?