CBQE-2260: add doc loader from folder

Example:
doc_loader_from_file.py -i cluster.ini -p bucket_name=default,path=/tmp/items/
all documents located under /tmp/items/ directory will be loaded to default bucket

Change-Id: I066ccf62d95e0c00205bd0849878d2e249a0084a
Reviewed-on: http://review.couchbase.org/37170
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>