Improving Wolfram Alpha search hit content

Making WA search hits contain
- the (parsed) input inside the "title" instead of just "Wolfram|Alpha", to better match other hit titles and to confirm correct parsing of input to the user
- the first output field that contains any text (skipping ones that are only pictures; this is usually the most meaningful "result" field) instead of the raw input as the "content", making it additionally possible to obtain WA computations from JSON API calls

fix xpath expressions for urban dictionary

closes #659

[engine] dictzone + mymemory.translated engine

details: http://stackoverflow.com/questions/18378720/bootstrap-3-with-remote-modal

Self information on later pages

Custom 404 message - fixes #317