Merge pull request #104 from SimonGoring/master

Adding the `download` and `download.list` classes.
Added classes, cleaned up data formats, built new tests.
* Added classes for `download`, `dataset` and `site` objects.
* Added 'time accessed' for datasets and downloads.
*  Created `print` methods for `site`s and `download`s.
* moved parameter checking to `param_check` for all methods to reduce clutter.
Merge remote-tracking branch 'upstream/master'