installable, dual py2+py3 compatibility, that fixes/tolerates some non-standard nodes

[notlaast]

I've started over with my pull request, to implement my changes on top of a rebased fork. I've implemented the following general changes

• the code is totally compatible with both py2 and py3
• the package can be installed via setuptools (that is, this works: "pip install ." and then "python -m vamdclib")
• the local registry has been fixed so that the CDMS and JPL nodes point to the correct URLs
• the registry works with some unidentified node that doesn't correctly provide a "referenceUrl"
• requests to HTTPS connections have been found to fail under some specific combinations of py2.7 + urllib2 or openssl libraries because of broken/unacceptable request contexts, and I've added an optional argument when initializing a new request so that this verification can be skipped

[notlaast]

By the way, my fork is based on several individual branches that focused on specific features.. please look more closely at my fork if you are interested in the details..