Ability to use APE as a library

[mthuurne]

When writing a system test, it can be useful to instantiate a mini version of a web application, load it with specific data and check whether it renders correctly. If a lot of cases have to be tested, or one test has to be repeated, it is more efficient both in execution and reporting to be able to call APE at an API level instead of having to build command lines and parse and combine output.

In particular if we're going to support POST requests at some point, it will be worth doing multiple runs of APE on the same starting situation, since the outcome can be different depending on which actions are picked.

Note that this of course is only an option if the system test is being written in Python.