Make it easier to select an introspection type.

Currently autopilot can auto-detect the introspection type to use when launching applications if the application is a dynamically linked binary. The current mechanism for overriding this is by overriding a method in AUtopilotTestCase. This is cumbersome because:

1) It applies to all the applications launched within that test.
2) It's a lot of typing

Ideally we need a keyword parameter to launch_test_application that allows the user to specify the application type - the default value should be 'Auto'. This means we can then consolodate the code in bin/autopilot and the code in autopilot/testcase.py. We should probably keep the old mechanism as well, in case the user wants to specify the introspection type once, for all applications launched.

Don't forget to update the documentation, including the porting guide!