Update cleaning of dataset (#72)

- Change commute times >300 minutes to NA
- Change minimum mortgage to $1000 and maximum mortgage to $1000000
- Move data dictionary into `clean-data/` directory
- Change minimum student debt to $1000 and maximum debt to $500000
- Add changelog to clean data README
- Remove missing data encoding information in README
- Add example exploration in clean data README
- Add figure of distribution of ages in dataset
- Clean data for children to make sure number of children and yes/no answer to
  having children is consistent
- Fix spelling mistake in `IsReceiveDisabilitiesBenefits` (original:
  IsReceiveDiabilitiesBenefits)
- Use `ungroup()` command in `time_diff_check` because of `dplyr` version
  changes
- Separate polishing steps for podcasts, resources, and so on to make it easier
  to see what is being polished
- Update survey data dictionary description with details on the two datasets
  and parts of the survey
- Update survey data
- Update version numbers for R packages

Author: erictleung