Merge pull request #142 from sbesson/118_chdir_decorator

Move cd() call to the low-level wrap-call() function
Merge pull request #136 from sbesson/130_pytest_fixtures

Use more pytest fixtures
Refactor filters definition in merge commands

- Define filters as empty dictionaries and fill them dynamically
- Use setdefault() and get() across the board to initialize non-existing keys
- Fix corresponding unit tests
Refactor filter parsing logic in filtered pull request commands

- Fully separate filter parsing from filter logging logic
- Create _configure_filters() method and
- Let TravisMerge from FilteredPullRequestCommands
- Move key/value and hash filter parsing to dedicated methods
- Refactor logic to include submodule PRs (using submodule name as filter key)
- Fix submodule filter inclusion/exclusion unit tests
merge master -D org -I include -E exclude: PR 143 (Merge commands filter refactor)