Change resource file API

[cbaatz]

Expect the routes export in resource files to be a function (parameters, log) { return <routes>; } instead of an object. Requiring a resource file should have no immediate effect. Allow resource files to export a description so we can list all available resource files with their descriptions. This also allows us to remove the knit global variable. Could allow routes export to alternatively be a routes definition object that can't access the knit parameters.