add indexing of datasets whilst harvesting
Merge branch '2.0-dataset-sources' of github.com:okfn/ckanext-harvest into 2.0-dataset-sources
fix gather_stage if there is a previous job

change check on gather stage to check for changed packages since
last job instead of current harvest job's gather_start

fix attribute look up bug

fix print_job to print 0 gather_errors instead of key error