tests: Require INVESTIGATE_KEY variable be set
Refactor

* Remove endpoints that the enterprise API doesn't have
* Comment out parallel methods for now
* Fix the request header
Remove 'Get' from method names
Add RRHistory() + tests
Add Categorization() + tests
Remove all parallel stuff for now. Can add back later.
Update method doc strings
Remove Score() and tests for parallel methods
Add DomainTags() + tests
Refactor: All methods return interface{}
Add LatestDomains() + tests