Add support for generic classes

It would be great to be able to do runtime type checking of generic classes, e.g.:

```python
from typing import Generic, TypeVar
T = TypeVar('T')

class A(Generic[T]):
@typechecked
def __init__(self, obj: T):
self.obj = obj

class IntA(A[int]):
pass

a = IntA(7) # good!
a = IntA('foo') # bad!
```

I sketched out something like this already: https://gist.github.com/chadrik/54d7b3d051839f90983676a47368526b