kgb is generally pretty straight-forward to use, but it's possible for

things to go wrong. In the past, failures to unspy could cause confusing

and hard-to-diagnose errors in other tests. Incompatible function

signatures could also result in errors that weren't clear.
This change aims to fix this by making some important checks when spying

on a function. The arguments provided are now checked in order to make

sure they're compatible with the spy, and are in the states required.
If a function is passed in that's already been spied on, an error will

be shown telling the user that the function has been spied on already,

and will show a backtrace of where the spy was set up, helping to debug

problems.
The call_fake function's signature is checked for compatibility with

the main function, in an effort to prevent future calls from breaking.

If the functions are incompatible, an error will be shown with the

functions in question, along with their signatures.
This should help a lot with the new signature requirements in kgb 2.0.

Unit tests pass on Python 2.7, 3.4, 3.5, and 3.6.
Tested against our other main codebases to make sure valid signatures

were valid, and invalid ones were caught.