Implement MARC fetcher and mappers

[lthurston]

Includes:

• MARC fetcher (for old-fashioned machine-readable, MARC)
• Mapper for ucb_tind, at the time of writing in a very early state

Notes:

• The code for the legacy ucb_tind mapper is nearly as convoluted as MARC is archaic. This represents an attempt to come at the mapper from a more developer-friendly perspective (a goal, as I understand it, of Rikolti generally). However, it also means that the rewrite may take some time to make feature complete.
• Class structure is not final. Specifically, ucb_tind, should be a subclass of OAI, maybe? And, development is happening in the ucb_tind mapper, but some of the word might apply to the marc parent class. This work is more a POC for the approach to the mapping.