Implement better credentials storage system

[katrinleinweber]

Currently, the credentials are saved in .Renvironment which is not ideal. In the long-term, a more secure way should be implemented. The overview by RevolutionAnalytics has some potential options, but:

• [x] check taxize's way of storing credentials (see also #34)
  • https://github.com/ropensci/taxize/blob/d23f2324a8d71806178a2ab55d4c4b73452aab89/R/getkey.R#L70
• [ ] try keyring, because it fits BacDiveR's use-case exactly
  • probably requires a different approach to Travis, see #101 & #103

Alternative: convince BacDive admins to drop auth.

Related to #2 & #6.