Fixed unit test, a few changes, and documented the methods

[vpenades]

After a later review, I found I left some garbage in the unit tests, which I removed in this pull request.

Also did some cleanup, and swapped two arguments of the Buffer and Image loader methods, so they're more consistent now.

Finally, added some documentation. Of special interest is the remarks explaining how to use the external dependency solvers.

[vpenades]

Thanks!