Added collect_dependencies feature.

[ftesser]

The purpose of this PR is to add the possibility of automatically collect dependent tests.
Is similar to what proposed in #37, but differs from these points:

• the behavior is automatically spread on all tests (no need to add the mark parameter collect=True)
• the behavior is enable/disable by a configuration that can be set in the ini file;
• the searching of dependent tests is recursive;

The configuration option of this modality is set to false by default, so pytest-dependency does not change its default behavior.

This is an update of PR #56; this update contains these new points:

• collect dependent tests of named dependency (@pytest.mark.dependency(name="a");
• collect dependent tests of classes @ydirson ;
• collect dependent tests of parametrize tests;
• a fix a typo in tests/test_04_automark.py;

The PR contains new tests for all these points.