Parallelize loading of datasets based off each file

* Trying parallelizing within each file first, which had good success
* Success when using pmap, which seems wrong
* Using agents, ran into issues on large datasets
* But agents worked well with multiple files

Idea for future:

* Parallelize by file if multiple files
* Else parallelize within file
Use clojure.data.csv instead of clojure-csv
Parallel loading of datasets is now the default
Load records in batch instead of one-by-one
Merge pull request #22 from cndreisbach/parallel-load

Parallel loading of datasets