Replace array with Base.convert

This pull request deprecates the `array` family of functions and replaces them with new methods for `Base.convert`. It maintains all of the same functionality, while bringing us closer into sync with Base. A similar pull request for DataFrames should be ready tonight.
Isn't this redundant with https://github.com/JuliaStats/DataArrays.jl/blob/jmw/convert/src/dataarray.jl#L608-L616?