Added mock resetting functionality

Now we can reset mocks using reset(mock); function.

Example:

let mockedFoo:Foo = mock(Foo);
let foo:Foo = instance(mockedFoo);
foo.getBar();
verify(mockedFoo.getBar()).once(); // getBar has been called once
reset(mockedFoo); // reset mock call counter
verify(mockedFoo.getBar()).never(); // getBar has not been called after reset