#12 Add type and weights to Resource links

[Kevin Black]

Add type and weights to Resource links.

In order to fully specify relationships between different resources we may need types and weights for the resource links, instead of merely types for the different resources on either side of the link.

For example, we might have a dictionary composed of word resources. Some words are roots of other words, some are derivations, some are senses of roots, and so on. Each word could potentially be related to multiple other words, especially when entertaining different annotation hypotheses for how the word relates to other words.

Trac #429.