Add documentation and minor changes #71

1. Add documentation to `sparql-hint.js` and `sparql.js` (mostly the
   former) to better understand what is going on.

2. In context-insensitive mode, make the object AC queries
   half-sensitive, that is, take the predicate into account

3. Improve suggestions for variable names, e.g., a predicate `hasAuthor`
   now leads to the suggestion of an object variable `?author`