Increating the time limit of the communication test.

In some testing environment it caused problems.
Pep8.
Use only new-style classes.
Changed JSONDecodeError to ValueError.

It is needed after moving from simplejson to json.
Adapt some pydoc to the correct format for raises.
Changed class to type when appropriate in the pydoc.
Timestamps are now in datetime.
Digests are really unicode strings.
Added logging explanation to two exceptions.

First is "wrong connection string", second is "service not specified
in conf". First still has several occurrence - we fixed only the case
in which the offer accepts a contest id.

I introduced a new exception, ConfigError, which is supposed to be
used for critical configuration problems, and is catched only in the
scripts' main.
Some pylint fixes.
Added some docstrings to Job.
.