[GPoS] VS topdown election algorithm

[badkk]

resolves #62

Unit tests changing includes:

1. phragmen_* -> topdown_*;
2. change nominators' voting strategy(equalization including candidates);

Covered black-box test cases

1. stake_limit should always be updated every new_era;
2. When the value of stake limit is 0, the validator identity needs to be removed #59 ;
3. Validatos without workload won't be remove cause phragmen mechanism #55 ;