add bin/mtest for running tests in multiple processes.
Update readme, describing bin/mtest.
Merge pull request #566 from 4teamwork/jone-threaded-testing

Parallel tests with bin/mtest
Referenceprefix manager: Made prefix-mappings of already delete objects removable.
Merge pull request #569 from 4teamwork/pg_fix_referencenumber_manager

Referenceprefix manager: Made prefix-mappings of already delete objects removable.
Added option to checkin documents without comment.
Pep8.
Fix typo in function name.
Bump default profile version to 4000.
Correclty remove replaced checkin action.
Refactor checkin, create separate classes for each possible action.

There are four different possibilities to checkin documents:
 - checkin one document without comment
 - checkin multiple documents without comment
 - checkin one document with comment
 - checkin multiple documents with comment

 Add a sepearate view and in case of the comment form separate form classes
 for each of above actions.
Update history, move entry to correct section.
Rename CheckinCheckoutManager method `checked_out` to `get_checked_out_by`.