Feature enable test assistant

This PR adds a test assistant for `enable` and `chaco` components. The `EnableTestAssistant` provides helper functions to simulate mouse and key press events and propagate them down the Enable event dispatch mechanism to help test `enable` and `chaco` Interactors. 

**In more details**:
- A `testing` module is added in enable that contains the `EnableTestAssistant`.
- The test assistant does not depend on a specific kiva backend implementation.
- The DragTool test case has been updated to use the `EnableTestAssistant`. 

**What is missing**:
- A few more examples showing more advanced usage of the EnableTestAssistant. However, I would prefer if these examples were added as real tests for enable.

**Notes**:
- Currently,  the testing module requires the Mock module to be available, but the dependency can be removed if necessary.