properely advertise xls
added missing import
add default storage type as 'string' instead of 'unknown' for convenience
fixed mismatches parenthesis
fixed typo
Updated changes
Added preliminary implementation of insert
Removed default log debug
Display call intention and retry in INFO level, actual call in DEBUG

Reversed information.
Added op: empty_to_missing (experimental)
Updated API doc
typo fix
implemented insert_into in pipeline; renamed basic nodes

SourceNode to StoreObjectNode, FactorySourceNode to ObjectFactoryNode
Renamed _ns_object_name to __identifier__
Implemented simple read-only FileSystemStore
added sql store metadata
SQL: Added exists() and better autoloading
added SQL ops: assert_contains and assert_missing
Added reason for probe assertion error
Added "before run" tests "needed"/"satisfied"; added name to pipeline.

test_if_needed() is a graph executed before pipeline. Test should be
satisfied to run pipeline, otherwise it is skipped without error.
test_if_satisfied() is a graph executed before pipeline. If test is not
satisfied, then pipeline is not run and exception is raised.