OPML export should be extended to include feed tags and face url

[jvasile]

The OPML spec at http://dev.opml.org/spec2.html says:

An OPML file may contain elements and attributes not described on this page, only if 
those elements are defined in a namespace, as specified by the W3C.

OPML can also be extended by the addition of new values for the type attribute. 
When specifying such an extension, following the example of this specification, say
which attributes are required and which are optional, and explain the roles each of the 
attributes plays, how they relate to each other, and what rules they must conform to.
There is a mechanism in the OPML Editor that is based on this form of extension.

Developers should, whenever possible, use capabilities that are already in use by 
others, or included in this spec, or recommendations or guidelines.

Our feeds have tags and face (hackergotchi) urls. We should export those in the OPML file planeteria should import from such an OPML file, as per issue #4.

[jvasile]

Actually, we can just adjust the template and not mess with namespaces. OPML spec does not prevent us from adding attributes.

[dtiburon]

Since I'm making major changes to the template.py file with the redesign, let's take a look at the OPML stuff once I'm done with it.

[jvasile]

We'll need unit tests for this. Marking "Testing"