Datetime code, test file named time.csv and test file path in test/io.jl

Unfortunately none of the tests are passing in test/io.jl so I cannot confirm that the code in this pull request is passing tests. But it works in practice (not an ideal situation, I know). This pull request is more aimed at exposing the code that will enable `readtable` to infer a date and construct a Datetime object versus simply returning a string as it currently does.