API-170 - find, count works on nested resources

Now, you can do this:

```ruby
district = Clever::District.find.first
district.schools.map { |school| school.name }  #=> ["name1", "name2", ...]
district.schools.count # runs a count query on the nested resource, not loading the actual elements in memory

student = Clever::Student.find.first
puts student.school.name # Notice that school is not plural! We can handle lists vs individual objects
student.school.district == student.district #=> true
```

Tests are coming.