Alpha version

Features

1. add copyTo command to export or copy the versioned graphs and associated datasets to another location.
2. add test command to test the existence and integrity of the versioned graphs and associated datasets in the data directory.

Improvements

1. use 3 level directory structure (e.g., data/ae/18/ae18bf...) instead of 5 (e.g., data/ae/18/bf/ae18bf.../data) (related to #9)
2. remove version index for individual datasets to reduce number of files in data dir (related to #9)
3. remove time stamp index for graph versions and datasets to reduce number of files in data dir (related to #9)

Bugs

1. include most recent crawl log when running preston ls.