kgb 1.0 changed how keyword arguments were stored and how they had to be

checked, requiring that called_with() and last_called_with() be

provided with the entirety of the positional and keyword arguments made

to the call. This wasn't really ideal, but was necessary with how those

checks were being performed.
This change fixes that and makes these calls a lot more flexible. These

functions now accept partial arguments. A subset of the positional

arguments (starting with the first one) can now be passed, and a subset

of the keyword arguments can be passed. If the subsets are found within

the called argument lists, it will be considered a math.

Unit tests pass.