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. 
Can we have a little more documentation about what `Krikri::Activity` is intended to be used for? Just a sentence or two would be great to start.