function decorators should use decorator to preserve signatures

[zzzeek]

example from #136:

In [1]: from dogpile.cache import make_region

In [2]: region = make_region().configure(
   ...:     'dogpile.cache.memory'
   ...: )

In [3]: import inspect

In [4]: def func(a, b, c=True, *args, **kwargs):
   ...:     return None

In [5]: inspect.getargspec(func)
Out[5]: ArgSpec(args=['a', 'b', 'c'], varargs='args', keywords='kwargs', defaults=(True,))

In [6]: cached_func = region.cache_on_arguments()(func)

In [7]: inspect.getargspec(cached_func)
Out[7]: ArgSpec(args=[], varargs='arg', keywords='kw', defaults=None)

[zzzeek]

didn't have the review hook set up, gerrit review at https://gerrit.sqlalchemy.org/#/c/sqlalchemy/dogpile.cache/+/996/