update source license comment to that of Elasticsearch core
Renamed ElasticSearch (camel case) to Elasticsearch

This includes renaming the acronyms from ESXXX to EsXXX
Aligning naming convention to ES 1.x
update year in Notice
update source license in hdfs sub-project
update license in hdfs module
add missing licenses
add license check task (though buggy)
rename Elasticsearch camel case to lowercase (in git as well)
add license header for build checks
update camelcase change in integration tests
update license check to apply only to main files