Remove dependency on the package 'humanfriendly'

[umayrh]

Is your feature request related to a problem? Please describe.
Generally, it's desirable to keep package dependencies as lean as possible. We use humanfriendly to parse a strings like "50b", "100kb", and "250mb" and convert them to integer values. We should be able to remove this dependency by implementing the parse_size() function in util.py (see here).

Describe the solution you'd like
An implementation of parse_size in util.py and corresponding tests in test_util.py.