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. 
`#get_record` is just the access point for a single record by id, in whatever way is the most efficient for the particular harvester.  I think for OAI, that's probably the GetRecord verb.

It seems likely that we will commonly want to generate a single `OriginalRecord`, and may play into some of the stories about doing updates for changed records only.