scan and scroll object for distributed search execution
#465 use guzzle for http transport
Follow code convention and updated changes.txt
added expiryTime and sizePerShard to __construct
added \Elastica\Search::scanAndScroll()
changes.txt update
Merge pull request #618 from milan/guzzle-transport

#465 use guzzle for http transport
Add pull request numbers to changes.txt file
Merge conflicts for ScanAndScroll implementation
Merge branch 'webdevsHub-ScanAndScroll'
Merge branch 'master' into fix-aggs-filter