general cleanup and modernization for the history module
928087 - serialized call request replaced in archival with string representation of the call request
fixed unit tests
Merge pull request #421 from pulp/jconnor-928087-pickle-regression

Jconnor 928087 pickle regression
Updating applicability api to accept unit_criteria instead of list of unit keys, updating docstrings and unit tests
Few minor corrections
Merge pull request #423 from pulp/skarmark-928509

Updating applicability api to accept unit_criteria instead of list of unit keys, updating docstrings and unit tests
Merge branch 'tmp-2.1.1' into pulp-2.1