Move data from vignettes to package data

[asbates]

• Moves the data sets in the /vignettes directory to /data-raw and include them in the package.
• Documents the data sets, but instead of writing out 200+column descriptions, adds a helper function get_dataset_labels
• Data sets in the /tests folder were left as is because there are some differences between these and the ones that were in vignettes.

closes #62

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[mstackhouse]

@asbates for all the data, can we update it so each dataset is prefixed with tplyr_? So for example, it would be tplyr_adae, tplyr_adsl, tplyr_adlb, etc.

The reason for this is that the datasets are likely named adae, adsl, adlb within the user's sessions when they're programming. So you don't want a dataset to be found in the namespace search.

[mstackhouse]

@asbates I tested out the pkgdown build and verified the site - found some changes that had to be made so I cleaned that up and updated the pkgdown yaml file. So once the checks all pass we should be good here.