Add support for fields parameter in get operation

Update the getDocument method to return the `fields` field instead of `_source` if available.
Allows to get a document from ES and specify the fields you want (See fields section at http://www.elasticsearch.org/guide/reference/api/get/)
Merge remote-tracking branch 'upstream/master' into patch-1
Add testGetDocumentWithFieldsSelection test case
Merge pull request #411 from loris/patch-1

Add support for fields parameter in get operation