Wait for a number of calls feature (#81)

Provides a family of **wait** functions that allow you to block test flow until a particular number of matching function calls occurred.
```
wait(Mod, OptFunc, OptArgsSpec, Timeout)
wait(Times, Mod, OptFunc, OptArgsSpec, Timeout)
wait(Times, Mod, OptFunc, OptArgsSpec, OptCallerPid, Timeout)
```