List expectations for same method alongside unexpected invocation error message

[floehopper]

See #167. The idea is to make it more obvious what's gone wrong. I'm thinking along the following lines:

foo = mock('foo')
foo.expects(:bar).with(1)
foo.expects(:bar).with(2)

foo.bar(2)
foo.bar(3)

unexpected invocation: #<Mock:foo>.bar(3)
- expectations for same method:
  - expected exactly once, not yet invoked: #<Mock:foo>.bar(1)
  - expected exactly once, invoked once: #<Mock:foo>.bar(2)
unsatisfied expectations:
- expected exactly once, not yet invoked: #<Mock:foo>.bar(1)
satisfied expectations:
- expected exactly once, invoked once: #<Mock:foo>.bar(2)

[floehopper]

See also #632.