Implement basic `import` command

[jamesaoverton]

The import command reads from an input file and inserts rows into the statement table. Eventually we want to be able to read all sorts of RDF formats, including triples, quads, and RDF*. The first thing we need is to read a TSV file with these columns: graph, subject, predicate, object, datatype, annotation. These should be added to the statement table with the next transaction ID and retraction "0". We want to add some triples about the transaction in a special graph (with the same transaction ID).