Update install docs for newer Debian versions

Tried to test this code out properly under testing+unstable as of 2014-05-06, and it fell down in a few areas. Substantive changes:

* libxml-dev (no such package, even in stable or Ubuntu) -> libxml2-dev
* libssl-dev is required now, and the "develop" phase will fail if it isn't installed
* Reformat instructions list to be better RST, and more consistent
* Make the test framework sound less optional :)