Added support for signature copying

[nickolasrm]

Made this modification because I had a code whose minimock couldn't help me easily:

def parse_varargs(f, *args, **kwargs):
    return inspect.signature(f).bind(*args, **kwargs).arguments

The code above represents a varargs parser I used in decorators. However, because the function f, already mocked, was passed to it, and because its signature was different, then parse_varargs output did not work as expected.

>>> def parse_varargs(f, *args, **kwargs):
...     return inspect.signature(f).bind(*args, **kwargs).arguments

>>> def foo(a, b, c=4):
...     pass

>>> mock('foo', copy_signature=True)
>>> parse_varargs(foo, 3, 4, c=5)
{'a': 3, 'b': 4, 'c': 5}
>>> restore()

>>> mock('foo')
>>> parse_varargs(foo, 3, 4, c=5)
Traceback (most recent call last):
  ...
TypeError: unexpected object <Mock 0x1f91fb498e0 foo.__signature__> in __signature__ attribute
>>> restore()

I hope this pr can improve this wonderful lib that helped me a lot!