Added configuration for PIT (mutation testing)

As PIT add a significant time to the build, I did not configure it to run by default. You can activate PIT by activating the withMutationTests profile :

mvn clean verify -PwithMutationTests
Added tests to command line argument parsing.

To make testing easier, refactored argument parsing and configuration management in separate classes. Single Responsibility Principle is also better followed, which does not hurt.

Adding tests, I found minor bugs in the verifications done on existence of configuration files, which I fixed.
As requested by @lookfirst, replaced all space by tabs.

At the same time, implemented a simple checkstyle configuration to make sure number of spaces used will go down. The maven property ${checkstyle.max.allowed.violations} is set to the current number of space violations. This will break the build if new spaces are added, unless you raise the number of allowed violations.
Added test to WatchDir to check if it work both with directories and directly with files.
Merge pull request #162 from gehel/improve-test-coverage

Improve test coverage