Add function for registering test frameworks

[jitseniesen]

The idea is that every test framework has an associated test runner, which follows the interface of the current TestRunner class. This class will be split in two classes: PyTestTestRunner and NoseTestRunner. The plugin has a function register_test_framework which takes the name of the framework and the test runner class.

This makes it easier to extend to other frameworks like unittest (cf. #4)