Wrap functions in class with decorator

The decorator should be simplified to a simple function, which should then return an object which wraps the passed function. Something along the lines of:
```
class Wrapper:
def __init__(self, func):
self.func = func

def __call__(self, *args, **kwargs):
# Get from cache...
# Or call self.func(*args, **kwargs)

def cached(func):
return Wrapper(func)
```

This simplifies the logic, and makes it easy to add methods on to our function (related: #538). For example, to call the function while forcing the cache to update could look something like:
```
@cached
def foo(...):
...

# Force update
foo.refresh(...)
```