GSoC 2019-Storing committed datasets in provenance directory

Hey folks,
We are done with 2/3 of GSoC 2019. The time surely flies.

For the last two weeks, I worked on the reviews on my PR for installation of datasets, added test for it. I also worked on implementing the command

retriever commit abalone-age -m "First commit"

In this case, the committed dataset will get stored in the provenance directory which can be set using the environment variable PROVENANCE_DIR. The provenance directory will store all the committed datasets in case the user does not provide a custom path for storing the dataset.

A log of all the committed datasets will also be maintained inside the provenance directory which can be accessed using the command:

retriever log dataset_name

It will show all the commits for a particular dataset in the following form:

retriever log abalone-age

Commit: Required results produced with updates
Hash: 78b2ec4a.........
Date: Sun Mar 24 21:45:21 2019 +0530

Commit: Required results produced 
Hash: 4ab32123.........
Date: Sun Mar 24 21:42:11 2019 +0530

I will work on implementing this command now.