Update Elastica\Filter\Indices

- New method addIndex allow to add indices one by one
- Indices may be specified by passing an instance of Elastica\Index instead of name string
Add execution option to Elastica\Filter\Terms
Improve coverage
Remove Elastica\Filter\Bool::setBoost

This function actually doesn't nothing:
- Boost parameter is for queries, not filters
- Even so, $boolFilter->setBoost(1.5)->toArray() returns array without "boost" index
Update Elastica\Filter\Nested

- Remove setScoreMode, because this option is for nested query, not nested filter
- Add join option
Update changes.txt