Helper methods

Single-letter methods are provided to make it easier to extract specific financial time series columns. The five column names that are supported include o() for Open, h() for High, l() for Low, c() for Close and v() for Volume.:

julia> using MarketData

julia> o(AAPL)
8336x1 TimeSeries.TimeArray{Float64,1,Date,Array{Float64,1}} 1980-12-12 to 2013-12-31
│            │ Open   │
├────────────┼────────┤
│ 1980-12-12 │ 28.75  │
│ 1980-12-15 │ 27.38  │
│ 1980-12-16 │ 25.38  │
│ 1980-12-17 │ 25.88  │
│ 1980-12-18 │ 26.62  │
│ 1980-12-19 │ 28.25  │
│ 1980-12-22 │ 29.62  │
│ 1980-12-23 │ 30.88  │
│ 1980-12-24 │ 32.5   │
   ⋮
│ 2013-12-19 │ 549.5  │
│ 2013-12-20 │ 545.43 │
│ 2013-12-23 │ 568.0  │
│ 2013-12-24 │ 569.89 │
│ 2013-12-26 │ 568.1  │
│ 2013-12-27 │ 563.82 │
│ 2013-12-30 │ 557.46 │
│ 2013-12-31 │ 554.17 │