Rbarlow export docs

In this pull request, I:

1) Reorganized all technical documentation, such that plugins and types are grouped together into the same files.

2) Changed some defaults to be in constants.py instead of hardcoded.

3) Documented the ISO plugins and types.

4) Documented the export distributor. For some reason, I organized this one as being outside of the yum_plugins.

This PR was made knowing that these docs are still not in an ideal state, so feel free to provide feedback as to whether this is what we do want for now, or if I should go further with it.