custom resolver signals

[jerch]

Shall fix #56.

This is just a proof of concept currently with hacked in signal dispatching for update_dependent.

TODO:

• apply data aggregation on all resolver parts including handler code
• find a good way to aggregate data with least mem usage / runtime penalty
• useful shaping of signal arguments
• single aggregation signal vs. more fine-grained ones or both

FYI: @mobiware

[coveralls]

Pull Request Test Coverage Report for Build 364

• 26 of 26 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 93.869%

---

Totals
Change from base Build 354:	0.08%
Covered Lines:	986
Relevant Lines:	1013

---

💛 - Coveralls

[jerch]

@mobiware Are you still interested in the signal feature? Ive found another use case with django-solo, which might benefit from update signals (it caches a single instance to reduce db interactions, which ofc would miss updates on the db values without any way to mark the cache entry as outdated).