Have schema_context operate as a decorator too.

[phalt]

Currently schema_context operates as a context manager but does not appear to behave as a decorator.

It would be nice if it could support both. This is possible from python 3.2 onwards.

An example would be something like:

@schema_context('tenant')
def a_function_that_i_want_to_run_for_one_tenant(*args, **kwargs):
    ...

[phalt]

Actually ya know what, I'll make a PR for this :)