API-162 - add count functionality. Also adds ActiveRecord-like find method

https://clever.atlassian.net/browse/API-162

Now you can do this just like `ActiveRecord`!:

```
Clever::District.find id # get element corresponding to a single id
Clever::Student.find ids # get all that match one of multiple ids
Clever::Event.find # get all elements
Clever::Section.find nil, limit: 20, ending_before: 'last' # supports parameters as per Clever API spec
```

As well as 

```
Clever::School.count # count total number of schools via a count request
```

Also, since the data structure that find returns for multiple queries implements `Enumerable`, you can do nice stuff like:

```
Clever::District.find.take 20 # get the first 20 elements.

# this is lazily evaluated by page, so it's not inefficient!
```

cool beans
assert_instance_of accepts only exact class matches, this works for all subclasses of `Integer`, hence the choice.