Support other testing framework

(Related to issue #10 )

Flacoco should support the same test framework that `TestRunner` (as it already knows, given a test framework, a) how to detects methods that are tests, b) to execute the test cases).
For example, `TestRunner` also supports AssertJTestFramework and GoogleTruthTestFramework
https://github.com/STAMP-project/test-runner/tree/master/src/main/java/eu/stamp_project/testrunner/test_framework/implementations

Note 1: `TestRunner` has a special runner for [jUnit3](https://github.com/STAMP-project/test-runner/blob/master/src/main/java/eu/stamp_project/testrunner/test_framework/implementations/junit/JUnit3Support.java). However, as jUnit4 runner is backward compatible with jUnit 3.x test, then I'd say that it's not necessary to have an special case for jUnit 3. Probably we would create some tests that executes junit 3 and see the output.

Note 2: Test framework neither supported by TestRunner nor Flacoco:
* TestNG
* ....
(In any case, focusing on jUnitX) we cover a large number of projects)

Start with the TestRunner test-framework implementations linked in the issue, then trace how Flacoco currently discovers and executes tests. Add support for the frameworks already handled by TestRunner, and add tests covering JUnit 3 execution and the resulting output.

Written by the indexing model from the issue text.