Fix a twisted test to work with Twisted 12.3.0 or better.

Twisted has updated trial to work better with unittest's test runner.
As a result, skipped tests are no longer reported as failures.  Since
this is the desired result, fix the test to no longer expect a failure
for the skipped test.  This was tested from Twisted 10.0 to 12.3.
Fix broken duplicate handler detection.

The old logic is broken (at least with newer versions of Python).  New
handler objects are created, and there is no mechanism for comparison,
so it resorts to using the object id.  This means that duplicate
handlers are not detected at all.  Let's check the baseFilename of
FileHandlers and the stream attribute of StreamHandlers as a way of
detecting duplicates instead.

With this change, the repeated log messages are greatly reduced.
Remove reference to deleted file from the docs

File doc_tests/test_coverage_html/coverage_html.rst was deleted in
commit 6347cd149f5f5f9f06ade7d635dc52245fd50124.
Document what --processes=0 does
Document default value of --process-timeout
Merge pull request #645 from tkf/processes-zero

Document parallel testing plugin options
Merge pull request #643 from mitya57/doc-fixes

Remove reference to deleted file from the docs
Merge pull request #641 from jszakmeister/no-multiple-handlers

Fix broken duplicate handler detection.
Merge pull request #640 from jszakmeister/fix-test-with-newer-twisted

Fix a twisted test to work with Twisted 12.3.0 or better.
Update distribute_setup.py to a newer version.

It helps to resolve some issues when installing under Python 3.3 when
there is no distribute package already available.
Correctly detect the coverage module under Python 3.3.

Python 3.3 has namespace packages (PEP 420) and the coverage support
area is being picked up as one.  As a result, when trying to run tests,
the current infrastructure believes that coverage is installed when it's
not, leading to several test failures.  Let's do an additional check to
make sure we have the real coverage package installed.
Merge pull request #657 from jszakmeister/fix-coverage-detection-for-3.3

Correctly detect the coverage module under Python 3.3.
Merge pull request #656 from jszakmeister/update-distribute-setup

Update distribute_setup.py to a newer version.
Fix #627: require at least 2 cpus for TestConcurrentShared

Anything less, and the tests will run serially and fail the test.