Add a Calls() method for call arg history

[Kangaroux]

This adds a Calls() method to the method mock object which returns a list of arguments the method was called with.

• Calls() is guaranteed to never return nil (will return an empty list if no calls are made)
• Methods which accept no arguments do not track their call params nor is there a Calls() method generated

[Kangaroux]

I didn't include it as part of the PR but I did test the case where a method has no arguments. If you create a new interface with a no-arg method and generate a mock for it, you can verify that there is no reference to callArgs or a Calls() method

[jesseroux]

@hexdigest Pushed changes

[hexdigest]

Thanks @Kangaroux !
I'll tag it and release a new version soon.