Feat: support gradle project in DSpot-diff-test-selection

[danglotb]

For now, DSpot-diff-test-selection can be applied only on maven program and prevent @pedrovelho to use it on Activeeon use case.

However, there are only a few things to do to make it executable on gradle projects:

• add a command line option to specify that we need to use gradle
• implements a way to execute Clover with gradle in eu.stamp_project.diff_test_selection.clover.CloverExecutor. One can reuse the code from Dspot-core.
• Update the documentation, i.e. the dedicated README to expose the command line usage and the new option.