Added documentation to RunLengthByte reader and writer. Did minor refac

Summary:
@public
Added documentation to RunLengthByte reader and writer. Did minor refac

Test Plan: Ran unit tests

Reviewers: kevinwilfong

Reviewed By: kevinwilfong

Differential Revision: https://phabricator.fb.com/D1427856
[DWRF] Add a testcase to make sure that setting offset in Reader.rows to a stripe offset will not skip that stripe.

Summary:
@public
I was a little nervous that this wouldn't work, fortunately it does, thought it'd be good to add a test case to make sure it continues to in the future.

The test case writes to stripes and reads them using Reader.rows setting offset and length to those of the respective stripes, and makes sure it reads the correct stripe and only the correct stripe.

Test Plan: The tests pass.

Reviewers: tejasp

Reviewed By: tejasp

Subscribers: hive-devel@

Differential Revision: https://phabricator.fb.com/D1427818