Observe validator changes

[AKolodeev]

I got in a situation when I needed to change validator depending on a condition. So I added validator observing.

[hueitan]

Thanks @AKolodeev Do you mind add some test cases to verify this feature?

[coveralls]

Coverage increased (+0.04%) to 93.656% when pulling 1a7f8f8 on AKolodeev:validator-observing into c92d8d7 on huei90:master.

[hueitan]

@AKolodeev

[AKolodeev]

@huei90, I am going to add tests next week. Thanks for reminding :)

[coveralls]

Coverage increased (+0.3%) to 93.958% when pulling 21dd54a on AKolodeev:validator-observing into c92d8d7 on huei90:master.

[AKolodeev]

@huei90, thanks for waiting :) I've finally done it!

[hueitan]

Thanks for this @AKolodeev !!! merge!