Merge pull request #7 from ropensci/master

Changing the DESCRIPTION file.
Create `download` class and add behaviour similar to lists.
Based on discussion in https://github.com/SimonGoring/neotoma_paper/issues/5 I have created a `download` and `download.list` class.  `download`s are single dataset downloads.  `download.list`s are sets of downloads.  `download`s now also include a timestamp using `Sys.time`.
Merge pull request #104 from SimonGoring/master

Adding the `download` and `download.list` classes.