Adds skeleton for pre- and post-processors.

This is an incomplete fix for the issue first reported in pull request #82, and is an alternate proposal to #131.

In certain places, there is just a useless function call in place of meaningful behavior. The documentation needs a list of the inputs and outputs for each pre- and post-processor for each type of HTTP request, as well as some examples. The tests are almost completely absent. We should probably incorporate some of the tests from #131.