Fix unqualified function call
Implement `wait for a number of calls` feature (Fixes #81)
Address code inspection comments:

- Move guards to `meck:wait`;
- Allow `0` call number in `meck:wait`
- Allow waiting from different processes on different or the
  same call patterns;
- Instead of starting a timer do garbage collection of stale
  wait trackers.
Merge pull request #99 from horkhe/feature/wait

Wait for a number of calls feature (#81 / #99)