Feature/harvester

Implements an abstract harvester with [lazy enumerators](http://ruby-doc.org/core-2.0/Enumerator/Lazy.html) for `records` and `record_ids`.  

Starts work on several other classes that will play a role in the ingest/enrichment pipeline. 

  - `SoftwareAgent` is a (currently empty) concern that will contain any info common to `Harvesters`, mappings, and enrichments.
  - `Activity` accepts an `SoftwareAgent` as its owner and a block to run, then tracks its start/stop times.
  - 'OriginalRecord' is a model for records as harvested from the partner.

An `OAIHarvester` is also implemented, with appropriately lazy enumerators. 
Not a hugely technical thing, but one of the first things I notice ;-)
Would anyone here or a potential contributor object to shorter line lengths?  I find that without a standard for line widths (like 80 ch, for instance) I can't be sure how wide to make my editor window, and I can't fit windows side-by-side when they're too wide.  Also, GitHub in this case makes me scroll this div horizontally to see the rest of the line.