Added HTTP/2 support and several schema and statement methods

Added HTTP/2 support.
Set 'version' config to 2 to enable it.

truncateCollection
You can now call truncateCollection on the schema handler.

$arangoClient->schema()->truncateCollection('users');

getCollectionDocumentCount
Returns the number of documents in a collection.

$arangoClient->schema()->getCollectionDocumentCount('users');

getWritesExecuted
You can now retrieve the number of affected documents after an executed insert/update/delete statement.

$statement->getWritesExecuted();