Plugins do not have their parameters checked in advance, so when we
try to aggregate parameters for a plugin, there's a chance it may
request an attribute of the FileProcessor that simply does not exist.

We catch this and re-raise it but we should also capture it in the
checker manager and handle it appropriately there as well.

Fix multiprocessing on Windows

*Related to:*  #184 

I'm still running into #179 with this fix but all processes start now at least.

BTW the current design is pretty bad regarding memory footprint and bootstrap time as you have to pickle the manager object for *every* subprocess. So this does not only fix multiprocessing on Windows. It improves the multiprocessing design on other platforms, too.

See merge request !97

Fix read_lines_splits_lines test for CRLF endings

This change makes the test pass for when files are saved with CRLF Windows-style line endings since those are included in various `.readlines()` methods by default.

See merge request !96

_job_count always returns an int

Trivial change, but `_job_count` always returns an `int`.

See merge request !95

As FichteFoll pointed out on IRC, this line could include Python 3s
less than 3.2 as well.