Fix/#166 uri support

[jawaff]

To summarize what I did:

• Lots of term changes because we no longer deal with just URLs.
• The JsonSchemaFactory.Builder is now able to be configured with URIFactories and URIFetchers. These factories and fetchers are able to be associated with a number of URI schemes.
• A default URIFactory and URIFetcher is configured for our custom Classpath URLs and standard URLs. (These can be overridden if the user wishes).
• The JsonSchemaFactory accepts a URISchemeFactory and URISchemeFetcher that are configured with the scheme->factory/fetcher maps. The factory and fetchers are chosen based on the URI scheme that is needed.
• The URL mapping feature should now work with URIs in general.
• URIs are created using the JsonSchemaFactory's URIFactory (which is the URISchemeFactory).
• A URI's stream is fetched with the JsonSchemaFactory's URIFetcher (which is the URISchemeFetcher).

I'll make any changes that are necessary.

[jawaff]

I think I've got everything working. Let me know what you all think. We can talk design and maybe figure out a better solution if you'd like. This is mostly just a proposition and I'm open to changing the approach.

[jawaff]

This pull request is meant for fixing issue #166. (I'm trying to get Github to link this to the issue).